You must find a Seed_Weed, these are located in a few places in the server. The best place to find them are when robbing houses in Grove Street.
If you find a Seed_Weed, you can then go to your House OR the free Motel you get when spawning and start planting it anywhere inside.
Once your Weed has fully grown, which takes about a minute or 2, It will ask you to “Pick Weed”, pick the weed.
You can now either keep the Weed for yourself, sell it to another player or if you have it equipped, any pedestrian you see will have [G] Sell Weed, which will allow you to go up to them, press G and start selling the weed. Sometimes they will call the cops, other times they may reject your weed. If that is so, just find another person, do the same thing until it sells the weed. The amount you receive will appear at the top of your screen.