I have a 25 day cycle my last period was the 18th of january if i'm possibly pregnant how far along am i?

Quite simple: If a woman is pregnant, and the first day of her last period was January 18th, then the number of days or weeks she is pregnant is just the number of days or weeks counting from January 18th. Quite simple indeed. No worries about the cycle being 25 days, compared to the textbook cycle of 28 days. In real life, 3 days makes no difference at all.