Will they still let me join the Church if I confess to an abortion?

You have nothing whatsoever to fear. The priest is there to absolve you of your sins—not to condemn you. That sin will be forgiven and nothing that you say during your reception of the sacrament of penance (as confession is formally called) can be revealed. We call this the seal of confession. Any priest who breaks that seal would be automatically excommunicated from the Church. In any case, your entrance into the Catholic Church will not be denied.