Birth control pills period changes?

Patient: I am about to take my birth control pills for the very first time…the problem is I am a bit confused…I know I have to start the day I get my period and then take them for 3 weeks and on the 4 weeks I need to take a break and then start them again but you shouldn’t take your pills while on the period so does that mean your period will change and I get them again after 3 weeks I started taking for the first time my pills and then my periods will come every 4 week?