How to tell your mum you're pregnant?

Hi,so im pregnant and im only 16 so yeah im a bit young it's not really what i want you know it will stop me being able to drink and go to party's night at my late teenage years because i will have a baby to look after.My boyfriend is very supportive and so are his parents.It was all an accident but we did use a condom and i found out last week that i am pregnant im going to see my mum this weekend and im 4 weeks along now,any ideas on how to tell her please? xx