I agree with monkey--Pro Tools is a great program for sound effects editing. But like he said, you can't "make" sound effects without having a ton of source recordings. You can either create these source recordings yourself (in a process commonly referred to as foley), or you can acquire them from a large sound database.
Large sound databases are almost impossible to find online, so I suggest asking your local school if you can pull some sounds. Keep in mind that complicated sound effects can have dozens of sounds involved, so you're going to want a large selection.
Another way to find sounds is simply to record them yourself. My uncle runs a
property maintenance company, so we get to travel outdoors a lot (we live in Ontario, Canada). I've gotten some really good close recordings of sounds like running water and leaves rustling.
With that said, you're going to need to proper microphone to record different sounds in the field. Bird recordings, for example, need shotgun mics for the long distance, and a satellite-shape attachment to better capture the sound.
Anyway, good luck!
"No matter where you go, there you are."