The female fish will find or make a nesting area (here in Florida, in freshwater you can see rings near the shore, and those are tilapia nests). The female will then find a mate and bring him to the nest. The female will drop her eggs in the nest. Then the male drops a cloud of sperm on amd around the nest, which fertilizes the eggs.
Edit: I bet this answer will be removed. Every single time I have answered in the most simple way (like you would to a 5 year old), it gets removed.