How do I go about getting my boyfriend citizenship after we are married?

You would petition for an immigrant visa for him. However, for him to be able to adjust status to permanent resident, he must have entered the country legally. If he did not, then he would have to go back to his home country, but would face a 10 year bar from the US upon leaving. A waiver can be requested for hardship, but no guarantee on approval.

If you are a US citizen or lawful permanent resident, you can file an I-130 petition after you are legally married. Whether or not your new husband can apply for lawful status depends upon many factors, so he should consult and immigration attorney.