Mother claimed her child and filed her taxes before me. I had child all year what can I do. Fo get child tax credit?

The IRS cares about physical custody.   If the child lived with you all year then you can claim the child----but you are going to have to file by mail.  Mother has already put the child's SSN in the system, and that will block you from e-filing with the child's SSN on your return.   

Print, sign and date your return in ink, and mail it in.   The IRS will sort out who could claim the child---and if mother could not claim the child she will re-pay any child-related credits received.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.