I have a question: does a man who converts to islam at 20 have to get a circumcision
Circumcision is not obligatory although there is disagreement about it (see this previous answer). So they do not have to get a circumcision, but most people would expect him to do it at some point.
How if I marry a converted man & he haven't circumcised yet. I mean, I've read it somewhere that u can't get married before circumcised first. Like, can we do the Ijab qabul first & few days after that the husband will be circumcised. Well, people who get circumcised need other people to look after them right? So what I try to say is I want to look after him during the recovery but obv it's haram before the marriage. That's why I want we get married first so I can look after him
Male circumcision is a matter of difference among scholars. The Hanafi and later Maliki opinion, and the opinion of al-Hasan al-Basri, is that it is recommended but not obligatory, while the Shafi`i and Hanbali view is that it is obligatory. According to the modern Egyptian scholar Yusuf al-Qaradawi, there is no conclusive evidence in Islam to make it obligatory, but that it is close to being obligatory and that it should not be abandoned by a society as a whole.
In your case, you can do whatever is most convenient. Here is an answer from a Mufti in Trinidad and Tobago saying:
Being Uncircumcised does not prevent you from becoming married. There is absolutely no statement that it is haram for a Muslim woman to marry a non circumcised man. When you intend to marry, it is good to tell your spouse that you are not circumcised, so that this does not create problems in your marital relationship.