Ehh---Music Theory for Techno or Trance?
Actually, I think you just need to know how to play an instrument, but of course, knowing music theory would make it way much easier.
What you really need is knowing about music structure.
about the techno sounds, you can download synths for free at VST4free, google it
A VST its the virtual instrument that will aid you with the creation of your music in FL, what you do is downloading the file and then you cut it and paste it to the "VSTplugins" folder.
after that, you go to FL, go to Channels-More-Refresh-Fast Scan
and your VST will be redlighted!
and well, for the techno sounds and stuff, you just got to search in the website, I recommend Vanguard.
just search, you will be amazed by the variety of sounds people can think of !
Wish you the best of luck.