Taking sounds from Freesounds isn't a problem, but you must learn how to give them a personality.
I personally like to use Reaper, a program that is easy to understand and free. With it you'll be able to mix several sounds to make more complexe ones. When I have to make a sound, I always put at least three different sounds in Reaper, but I can even put twelve for some short effects! Nevertheless I never have any problem to keep track of them, and I'm not even that good with it.
Make yourself a list of sounds you appreciate, musical instruments and other stuff that can be useful like woosh effects.
Avoid sounds that are already heavily edited as they wouldn't be very interesting for you to learn.
And just start toying around with it.
One trick that can help you start is doing the following :
- Double-click on a sound-track on Reape.
- Uncheck Preserve pitch when changing rate
- Push Alt, then stretch the sound
That'll allow you to make easy pitch variations from the same sound.