I am U.s. citizen, what do I need to get her to come to visit from Mexico.

Start with paying a vist to the INS. THEY have to apply for a VISA to visit... not easy as there are quotas... if they want to immagrate , they have to apply for a different VISA.. even longer lines and different quotas... and don't think about getting cute and coming as a tourist thinking you can change it later.. you can't.

Expect it to take years... and expect it to cost a LOT of money... your best bet is to hire an immigrations lawyer.

To visit, like come here for a few weeks and go home ? Just a Visa, they will apply at the US embassy in Mexico.

If they want to marry or work, it takes other visas

If you are talking about a visit, your relative must apply for B1/B2 visa (tourist visa/visitor's visa) at US embassy.

She may or may not be eligible for immigrant visa... depends on relationship with you.