What steps should be taken if you want to marry a girl who was brought into this country as a child and has never applied for citizenship although she's been here for 10 years?

If she is 18 or over then she must apply for citizenship herself with her parents proof of citizenship. If she is under 18 then her parents can apply for her and if they are citizens it is just a matter of automatic approval as long as there are no criminal records found in the background check.