How to quickly grow an army and maintain it?

Solution 1:

http://day9tv.blip.tv/file/3732340/

Watch that video. It will answer all your questions.

To highlight the main points he makes, it's all about macro play. Make sure you are always building a new worker. As soon as the last one finishes, build a new one. If you have way too many, expand and split them up.

Make sure you have enough production buildings so that you can build your army quickly enough. You should never have to queue units and always be able to spend all your money (which should be plentiful since you're always building workers). Hotkey all your production buildings and get in the habit of cycling through them to see which are idle, and then make sure you un-idle them as soon as possible.

Make sure to watch your supply and build quickly enough so you never get supply blocked. You always want to be building new units.

Solution 2:

Your early army doesn't need to be of any significant size. You can harass quite effectively with the right units and in fewer numbers. While doing so you can continue to build up your economy and your mid game army.

As Terran, highly effective early game harassment units would consist of either Hellions or Reapers (depending on your opponents race) Hellions will tear right through drones and zerglings. Reapers are very effective against probes (protoss). Also, keep in mind this involves a lot of micro, its not simply send your few units in and watch. You will want to constantly harass and run away. With reapers you can hit his mineral line and when your opponent brings in units to counter, run away and attack some where else.

Doing this will also give you a huge advantage on what build your enemy is going so you have more than enough time to make sure you have the right unit composition to counter.

While this is going on you will want to continue building units, scvs, buildings, mules, etc. Don't forget about your base and focus only on attacking.