Welcome to MustangWorks!!! Really what I have found to be the best method is just hang out here on the 'boards. Read everything and listen to everybody. There's several Mustang modification books on the market too. If you're new to working on cars (as in under-hood mechanical stuff), pick up a Haynes manual. I still refer to it myself time to time.
If you want biggest bang-for-the-buck parts, I think everyone here will tell you rear end gears are THE place to start. Closely following would be bumping the ignition timing (along with higher octane gas), getting a K&N air filter, and doing regular tuneups including oxygen sensors & PCV valve. After that it'll take heads, a cam, intake, exhaust, etc. Good luck in your quest for more power.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch