Not all therapists suit all patients and some times finding the right one is a difficult process. If we oversimplify things the basic thing they do is change your perspective. A lot of our emotional or mental problems may stem from our environment but many times we get ourselves into a mental loop that keeps us there. Ultimately nobody can "fix" someone else, everyone has to do that for themselves, but the job of the therapist is to help guide someone through this process and provide a different perspective, without just talking out of their ass, which any friend or family member can do.

The first and most important part is knowing what you're there for. "Wanting to feel better" is not enough, you have to have goals in mind. Some times if the source of the problem is unclear they can help you find what it is but a mistake many people make is expect too much from them without giving any feedback to them. It's a cooperative process, they can't just lecture you and make you feel better. A common problem is that a lot of people think they're "doing their part" by going to therapy, but other than going to therapy and venting they're not actually doing anything different in the rest of their time and lives. Going to therapy means being willing to try stuff out, making an effort, even if it doesn't always work the first time.

But when it works it can help you get yourself unstuck. It changes your perspective on yourself and your surroundings, helps you negotiate relationships with others better and help you deal with future issues better.