If I'm 19 years old and about to have baby from an illegal person, how can I make him a resident if he's here in the USA?

If he entered the US unlawfully, under the current immigration laws you cannot legalize him or help him legalize even if you marry him.

If you marry him, and assuming he is otherwise eligible to adjust status, then he may be able to get a green card. To know for sure, I suggest that you both consult with an experienced immigration attorney

Your question requires an attorney consultation. It is not a simple question that can be answered on this type of forum. There are many factors that would need to be considered and evaluated. At the very least, you would need to marry him to petition him for a green card. I strongly suggest that you contact an experienced immigration attorney for a face-to-face consultation and give him/her all of the facts surrounding your situation. (S)he would then be in a better position to analyze your case, advise you of your options and, if possible, make sure that the necessary paperwork and documentation is presented so as not to delay the process.

You would have to form a relationship first that would make him eligible for any immigration benefit by marrying him. However before you get married you both may want to see an Immigration Attorney to see what options are available and help you move forward.

There is a quite complex procedure in place to help the father of your child to obtain residency in the United States in case you marry him. I would highly advise on retaining an immigration attorney to assist you with this matter.

Maybe he's eligible for DACA. Additionally, if you are a USA citizen, and you are married to him, you can file an I-130 visa petition to get the process started.