Game modding boils down to being able to load cheats and change variables within a game at your will. There are various options for modding PS3 games. If you are just looking to use simple cheats for an offline game, it may be easier to use Artemis. Note that modding online games will put you at a higher risk of a ban.
A second option is to use a pre-built set of mods, often packaged as a "mod menu", which comes as an EBOOT, SPRX, and/or a RPF.
Check to make sure that your desired mod is compatible with your firmware type (CEX or DEX). It is recommended to add ".bak" to the end of your original file names in case you need to revert back. The process for each mod is as follows: