I need some help, my baby sister's birthday is coming up and i wanted to give her a fish tank with some fish

You could make a sponge filter. They are very easy to make. Do a google search for DIY sponge filters. You could catch and keep some minnows for quite a while and then release them back into the lake or river when they get too large for the tank. She would probably enjoy watching them swim around. Dace minnows are good and they grow fairly slowly, although I don't know how you would pick a species when you are catching minnows! Just net a few minnows and get some aquatic plants and put them in the tank for her. When the fish start to get too big for the tank, release them back into the stream and get her some more minnows.