I'm new to GOG and I am having trouble finding out how to install mods and patches on games. I especially want mods and the unofficial patch for Bloodlines because I read that it doesn't work well without them. I see that it already comes with one patch installed, but what if I need more? I searched the discussions but didn't find what I needed. Thanks.

Basically what you need to play the game mostly bugfree is Wesp's base patch which comes with the GoG version of the game.
If you want the plus version of the patch or maybe try a mod for a second playthrough, you can find them on ModDB.
Just keep in mind that for bloodlines you can only play one mod at a time. The exception to this rule are skin and music replacers.

Most of the GOG forums for specific games have a sticky about modding (or at least the ones for moddable games do). If you read that sticky then you will become an expert in the field of modding that game. If something confuses you, just post a question in that topic. Enjoy!