In some very extreme cases, there may be more modeling agencies in a given area than there are models - or even potential models. This is because many people have figured out that they can make money by preying on people who have a dream and are willing to do anything to make that dream come true, even if it means paying for the privilege of joining modeling agencies which are no more likely to find them work than buying Cosmo and reading through the classifieds will.
Not all modeling agencies are parasitic money grubbing sneaky fakes, but many many many, are. They take advantage of the ignorance of mostly young women in order to create a roster of talent that is rarely used and constantly told to just be patient or that it's just that this that or the other thing is wrong with them and as soon as they get a tummy tuck or cosmetic surgery they'll start working right away. So be aware that modeling agencies must be researched before you sign up, and you should never pay to sign up.
In the case of a few respectable modeling agencies, you have nothing to fear. They will look out for your best interests, and never ask for money until you have been paid for a job that they helped you get. Modeling agencies can be a great help on your career path, so don't delay doing the research and figuring out which one is best for you.