go into your soma folder, and look for the hpl3 level editor, and open it. also it might take a while loading up so be patient with it since this engine can be pretty cranky sometimes.

Thanks! What programming language do I need to learn for modding?

Well it's like c++ which if you haven't done any scripting at all before then it'd be hard since that is supposed to be a hard programming language to learn first. Anyway Frictional Games uses one called Angel Script i believe, but it's easy to pick up on, just takes a little work. For the most part I just looked at their work in SOMA and other people's too and experimented a little.