If my girlfriend and i had a baby in 2018, and she is going to claim him. Do i need to say I had a child in my taxes, or do i not write anything at all?

If all of you live together but you are not married, then only one of you should put anything on your tax return about your child.  One of you just files as Single.  Which one of you had the bigger income?  You should actually try the tax returns both ways to see which way comes out better for who should claim him.  The one who paid over 50% of the household expenses could claim him and file as Head of Household.

Thank you!

Do not write anything at all.  The interview is confusing (it's designed for divorced parents) and frequently leads to mistakes by unmarried parents living together.