Right now, we’re in a bit of a transitional phase between two iterations of the topics system, so it is fairly clunky. There is a master list, but it’s not readily available as far as I know.
So I’ll tell you three ways that I can think of to add topics.
1) Find a user who shares similar interests with you, and go to their “Topics” tab. Some users have more filled out topics pages than others, so your mileage may vary depending on who you pick.
2) Find a question that interests you, and add topics from it.
3) Go directly to the Topic page for the subject you’re interested in (http://www.fluther.com/topics/X, where X is the thing you like. e.g. computers). You will also be shown 3 related topics there, as well as questions using this topic, and members who have picked this topic for their own list.
There’s a pretty massive selection of topics, I would be surprised if any interests of yours wouldn’t be on there. Unless you’re very specific. Like, for some reason, Symphonic Metal doesn’t count as a topic, much to my dismay.