1. Ask around for a contact. Be discreet-- only ask people that you already know smoke weed, and only ask through a private medium (like texting or a discreet conversation).
2. Get a number, preferably of someone you already know. Agree on a meeting time and place that isn't too sketchy. Don't tell them you're a noob if they don't already know, because many people will unfortunately try to rip you off.
3. Ask how much it is for a "g" (slang for a gram). Really good weed will go for about $20 most places, but since you are obviously not yet a good judge of quality if you are reading this guide, ask for "10 a g." Buy either a "dime" (one gram, for ten dollars) or a "dub" (two grams, for twenty dollars). Keep it simple.
4. CONTINUE to be discreet during the deal.
5. If you can, watch them weigh it out for you.
6. Don't just take the bag, pay, and run. Smell it first. Weed does NOT smell like oregano. It might smell "skunky" or any of a variety of other scents, but it should be a pretty strong smell. Take a look at it too-- orange or purple mixed in with the green is a very good sign, as is visible little crystals, but both these traits are unlikely for average-quality weed. It shouldn't be too dried up or too wet. As you gain experience you will gain a sense of this.
7. Don't settle. The underground market is used to bargains. If you think you're not getting what you're paying for, refuse it or express disbelief about the price.