Research the contracts publishing companies offer you. You, as a self-publishing author, should have the ability to keep all rights to your manuscript. Never cede the copyright to your manuscript to a company. You should be able to break contracts without penalty if you find the publisher's work unsatisfactory. Do not lock yourself into a contract.
Price is important for writers with limited budgets. Research the cost of self-publishing companies. Look at what services they provide for the cost. Are you able to set your own sell price for the book, or does the company set it for you? Publishers that set the price do not have your financial interests in mind.
Research what services, including customer service, the company provides. Find out whether the business offers placement in bookstores or with on-line retailers. See if the company leaves it to you to find venues. Availability of professional services, such as creating book covers and tutorials on self-publishing, is useful, too.