Do drugs (weed specifically) by going to the weed field located north-northwest of the Alamo sea by Sandy shores. Go to weed plants and press E to harvest. Once you have the weed as a raw product, drive down into Los Santos and make your way down to Los Santos International Airport Los Santos Customs where to the left of the workshop you'll find a blue door. Walk Infront of the blue door and press E, you will enter the weed processing facility. Upon entering, walk to the tables Infront of you to your left (hard to miss) and stand on the green circle. Press E and you will begin to process your weed into cannabis. You get 1 Cannabis from every I think 3 or 5 weed. Once you are finished processing the good stuff ;), make your way over to Vespucci beach. On the beachfront you will find a weed shop that is only accessible during the day. Walk to the counter and press E to sell your product. Note that the money you receive is dirty and needs to be cleaned via a business owner. I heard the Casino up on bridge Street does a good deal as well a John Doe enterprises. These owners will swap your dirty cash for clean at an interest rate usually being 25%. - 5% depending on the amount you need cleaned.
Trafficking Weed is very high risk, you will be very prone to robbers and police so I'd recommend doing your runs with extra protection (Additional friends/Gruppe6).
And by the way, welcome. I look forward to seeing you on the right side of the law :).