How to know when h have started to dialate

marcia

I went to the dr today an I have been having hard contractions and I lost my mucas plug but my cervix is closed ? When does it usually open and how far along was u when u started to dialeted he is testing me for preterm labor but idk I just wanna make sure he stays in till he is ready but I’ve been in so much pain I’m ready for him I’m only 34 weeks