I would like an easy way to merge some photos together to make some kind of collage.?

Or a wall poster. I have one photo of two girls with a cheetah inbetween them and i would like to get rid of one girl. Can i replace her with something else or cut her out and fill in the background so it blends in. Know what i mean? I seen loads of videos but as soon as they say "its so easy,all you have to do is etc etc etc "then i know i aint going to be able to do it.. Any help out there please.