If you use windows, windows movie maker is easy and basically does all the thinking for you such as music, choosing clips, photos etc. It should be on your machine already1 Mac has similar stuff, but I am not sure what it is called.
There are also online sites, like
http://www.onetruemedia.com/, that you can use to help you put together montages etc.
It is very easy, it has to be or there wouldn't be millions of montages on youtube!!
Let us know when you make one that you like, we love seeing gymnastics!!!