Nice strategies but here's some tips and strategies that's helped me so far against the AI on SC2. This is just to help get you off of the ground:
Terrans: First build up to the 11 SCVs. If the map has the normal 8 Mineral Fields, try it this way, put one SCV to one Field then one extra SCV to a random Field, but perferrably to one of the border Fields, which are the top or bottom Field of the bunch for your memory purposes. Cue up another SCV before the 11th one is done. Now, you've gotten two SCVs doing nothing, and should be somewhere around 150 to 200 Minerals at this point in the game. While the rest of your SCVs mine Mineral Fields, you have one of your idling SCVs build a Barracks somewhere easily accessible near an exit such a ramp or bridge. Now have your other SCV build a Supply Depot nearby. It shouldn't be too bad of a placement considering they can "flatten" nowadays, or go underground. At this point you'll be still collecting Minerals with your miners. Your two building SCVs should get done with their buildings roughly at the same time. When they're doing, set them both to build one Barracks a piece in a triangle fashion, but not next to each other. Build them a few tiles away from each other for their little buildings next to them. This is so you can have two Barracks with one Tech Lab a piece and one Reactor. The purpose is so you can produce two Marines at a time with one Barracks and two Mauraders, Ghosts, or Reapers, or whatever combination you desire. If you're on Very Easy or Easy, you can actually get away with building more SCVs to start mining Gas from your Vespene geysers soon after building two Tech Labs and a Reactor for your three Barracks, then start building a more complex force after a dozen or so Marines to help fill out Bunkers which should be place at the top of a ramp or your end of a bridge. The original SC and Brood War's Terran Bunker/Siege Tank combination even with the Missle Turret still works wonders in SC2.
Zerg: Set all of your initial Drones to Mining. But don't build any Drones till you've reached at least 150 Minerals seeing that they cost 50 Minerals a piece. Once you've reached 150 Minerals select all three Larvae and morph them into Drones. When they've hatched, set two of them to mine from the two empty Mineral Fields, empty being noone's mining from them, and the third to mine be the second in line to mine from a bordering Mineral Field, again for your memory purposes. When the first larvae spawns at your Hatchery, turn it into a Drone, and the next larvae into an Overlord. This'll be because you've hit the underling limit at that point in the match for yourself. Once the Drone hatches, set it to mine from a Field. You can directly set the Overlord's egg to have the Overlord to automatically go wherever you want it by right clicking on the Mini map or elsewhere, when it hatches. Now, wait for three more larvae to spawn at your Hatchery. Yea, it's paranoid working this way but pays off well for the most part I believe. While you're waiting for the third larvae to spawn your working Drones are still feeding Minerals into your coffers. By the time the third larvae spawns, select all three, and turn them into Drones. You should be roughly between 250 and 300 Minerals after turning them into Drones. By the time they hatch you should be around 400 and 500 Minerals. Now, select one of the newly hatched Drones to turn into a Hatchery. Now do the same with the second Drone. You can always wait for your Drones to gather up another 300 Minerals at this point, or settle for a Spawning Pool. I suggest for less experienced gamers, to settle for the Spawning Pool. At this point, you won't have the funds to pay for a Queen, and without a Spawning Pool, she's an impossiblity. So I'll split this path into two ways - Path One, you took my advice and went with turning your third Drone into a Spawning Pool. While your two new Hatcheries and Spawning Pool are transforming you continually hatch new Drones till your Hatcheries are completed buildings. In between the completed Hatcheries you set your new Drones to mine from the Mineral Fields. Once your Hatcheries are completed, you take three larvae and turn them into Drones. When they've hatched set one to be the fourth and final Hatchery in your primary base. The other two Drones are to be turned into Extractors. One Drone per Vespene Gas gesyer. Then at this point, you start building up a Zergling force as big as you can to fend off the first wave of attacks. You'll need to mutate three more Overlords after making your first few Zerglings after your two Hatcheries have made two or three larvae a piece. Your underling limit should be 22 by this time.
Now for Path Two - This is for the daredevils or more experienced gamers. This one you don't use your third Drone to mutate into a Spawning Pool, but instead another Hatchery, once your Drones gathered another 300 Minerals. It shouldn't take long at all. Once you've attained 300 Minerals, create the fourth and final Hatchery for your primary base. Most likely waiting for the gathering of 300 Minerals have taken enough time for three more larvae to spawn at your only active Hatchery and continually hatch more Drones till you have at least two Drones per Mineral Field.
Protoss - Like with the other two species, send all of your Probes mining from the Mineral Fields. Keep build Probes till you hit the 9 Psi limit. Hit that, you should have one Probe in queue still. Now, you take your 9th probe to warp in a Pylon nearby your Nexus. You should have enough Minerals to warp in another Pylon. Warp the next Pylon a few tiles away from each other. As in enough room from each other to overlap each other's power fields some. Now once both Pylons are finished warping in, your 10th Probe should of gotten done being built. Send that to mine too. You should be somewhere around 300 Minerals at this point. Now go to your first Pylon and warp in a Gateway anywhere you want in the Pylon's power field. My style's been adjacent to my Pylons. Once you've gotten 150 Minerals at least, warp in your second Gateway. Keep this up till you've gotten the Minerals to warp in two more Gateways within the power fields of your two Pylons. Once you've done this, at least two Gateways should still be in the process of warping in. Take this time to continually build more Probes to gather more Minerals. A bit after all four Gateways have warped in it may take a few more seconds for you to reach 400 Minerals. Once you've gotten 400 Minerals double click on one Gateway, assuming you got your Gateways within reach of each other, double clicking one Gateway will select them all, upon selecting them all click on the Zealot button four times in a row to warp in four Zealots for one Zealot per Gateway. Once they've warping in, you'll want to warp in more Pylons. I say two or three isn't going overboard at this early stage in the match. You can build more Probes to gather from Mineral fields till there are at least two Probes per Mineral Field, and keep warping in Zealots to fend off the first wave.
Yea, it sounds complicated, but it's not in practice. It can be nerve wrecking on harder difficulties to try out new strategies. So my best advice is if you're really that new to Starcraft, Brood War and Starcraft 2, go with Very Easy first, then level up to Easy. As a warning to all, there's a big difference between Easy and Medium. And even more so a step up from Medium to Hard. I presume Insane would be literally friggin' insane of an AI. By the way, the end of my little strategy thingie for the Terran side of things, yes, keep building enough SCVs to have two of them per Mineral Field. This wasn't to be advanced, maybe a bit or more so intermedietly or something, but just a path to give one a place to start with.