How to raise credit score?

My credit is awful because I have 6 hospital bills against me. Only about 2k total from 2015 and 2017. When I was married, I was a sahm and had no personal finances so I literally have no credit. Never had any bills or loans in my name. No credit cards. Me and my fiancé can’t buy a house together despite me working because my credit is shit. How can I raise it? And I can’t pay my bills off. I don’t ever have extra money for that. I go to school part time and work part time. Have two kids and pregnant. Should I get an unsecured credit card? Should fiancé add me to his credit card? Ugh help

Not that it matters I guess but fiancé’s credit is wonderful. Like 780. And we made about 60k/ yearly combined so we do okay... not great. We currently own a house but we have to sell and move within the next year.