Search online for publishing resources. Look through the larger sites that publish lists of reputable publishers and the appropriate contact information.
Subscribe to a few well-known publishing newsletters that announce book deals that tell you what publishers are buying and what types of books are getting contracts.
Look at examples of the type of books you want to publish, and pay attention to who published them. Note those names, and find their contact information online or in a directory that lists publishers.
Buy the latest copy of the Writer's Market, which changes significantly every year. Study the Writer's Market to get an idea about how a legitimate publisher operates and what types of communication and submissions are generally acceptable. Submit your manuscripts to the contacts listed.
Sign up for the online version of the Writer's Market, which you can purchase online or by using a code found inside special versions of the book. Check out the online version to get the most recent submission and contact information.
Hire an agent. Rely on your agent to send submissions to publishers, many who only accept manuscripts from agents.