Don't limit yourself to communities! Not to say communities don't get used, but most people post more frequently to their personal journals than to comms.

If you want to look for people with similar interests to the ones you listed in your profile, you can click on any linked interest. If an interest is linked, that means other people have the same interest in their profiles as well.

If you want to search for people or comms with other interests than those listed in your profile, look for the search box on your profile page. By default, it will search on interests, though you can search on other options as well by changing the drop-down menu.