The services offered by literary agents vary, some offer a great deal of creative input and editorial support whereas some will simply concentrate on marketing and promotion, but all literary agents should submit work to publishers on your behalf and will make sure that you are paid fairly. To find out which agent will work best for you it is always best to do a bit of research. Most agencies now have websites which will give you information about the background and experience of their agents, and will also often include details of their services. A reputable literary agent will not ask for a reading fee (a fee for looking at your work) instead, if they take you on, they will take a percentage of any royalties, usually around 15%.
Unfortunately finding a literary agent can be every bit as difficult as getting a publisher and many agents prefer to work with more established writers. Please note that agents will also expect work to be professionally submitted (See Getting Published for more information).
If you are lucky enough to be invited to an interview, you should treat this like a job interview and make sure you are prepared to talk about your work and how you can develop as a writer. An agent will not want to take on a writer who can only write one book.
Finally, it is worth making sure that you like and respect your agent. After all, if it is going to be a long and fruitful relationship, you will have to get used to each other!