Working as a session singer is a good way to get experience of singing in a recording studio. You'll also get to know music producers and if you do a good job, you may even end up featuring on a track that gets released. Many huge stars, including Whitney Houston, worked as session singers before becoming famous.
To get work as a session singer your best bet is either to offer your services to producers or songwriters you know (you may have to work for free at first to get some experience) or sign up to an agency. Here are a few that seem reputable: