just google "pregnancy calculator" then put dates in, it will tell you. and in anticipation of your next question, it's spelled G O O G L E and you need to put ".com" on the end.
Edit: Just realised I misread the date in the OP. More chance of me being a glitter unicorn than you being pregnant at this point in time. Please, please go learn how cycles and pregnancy work before trying to breed.
Your period has nothing to do with it (well not nothing...). It's ovulation that tells you how far along you are. The day of ovulation is considered 2 weeks of pregnancy. It's not possible to know you're pregnant before 3 weeks and probably closer to 3 weeks and 2 days...at the very earliest.
Yes, you need google. But I think it goes a bit deeper. Seriously, consider picking up a book like Taking Charge of Your Fertility and learn how your cycle actually works. Not being snarky.
Re: If I'm pregnant - my last period started on 1st jan 15 - how many weeks am I ?
and in anticipation of your next question, it's spelled G O O G L E and you need to put ".com" on the end.
Jan. Siggy Challenge; Exercise Fail