My boyfriend made love to me unprotected, and this was the first time he ejaculated in me

How likely is it that I could get pregnant after such accident that occurred for the first time?

Unfortunately, I have some bad news because it could happen that you got pregnant (it doesn’t matter that it was the first time). If 72-120 hours have not yet passed since your intercourse without contraception, you should visit a gynaecological clinic, where emergency contraception will be prescribed for you. If this period is already over, you should take a test at the time when your next bleeding is expected, provided that it is late.