First pregnancy and terrified 😳

Rebecca β€’
I'm Rebecca, I'm 21 and found out on Friday I am pregnant. I've done 3 tests after being almost 3 weeks late on my period, I didn't think anything of it because since having the injection my periods have been so irregular but was having bad cramps which would come and go πŸ˜•Β 
Is it normal to be absolutely petrified, but also feel so useless?!
Β I don't know where to go from here, do I ring my doctor or just wait? and how do I know how many weeks I am? I bought a clear blue test as shown in the picture which says I conceived 4-5 weeks ago but apparently you count it from the first day of your last period.. So am I 4-5 weeks or is it classed as 7?Β 
Sorry to go on πŸ˜‚ I just feel slightly lost right now but also secretly excited!Β