What do you do? Stop using - even just weed.
And fess up to your mom. And tell her you realize it was dumb and you will stop -- better to be up front with your mom than her to hear elsewhere.
Then show you are responsible -- help out around the house, do your homework and do well in school.
Basically now that you shown you are smoking weed -- you need to re-establish trust even if you were not the one to get your friend involved. You have proven by using you are not responsible so know you must instill trust again.