The free sound project is an online repository of sound files that fall under the Creative Commons License. The site is designed to make it easy to find any type of sound you may need for your game. Setting up a user account is free and gives you access to a large library of sounds. Although it is free, it still has support in the form of documentation and an Audacity Wiki.

Once you've acquired sound files, either from a site like The Free Sound Project or somewhere else, you will need an environment to edit the file. Audacity is a free cross platform sound editor. It has many features including recording, editing, splicing, pitch alteration, mixing and more. This is not difficult to start using and is great to have for prepping
sounds to be brought into your game.