I'm sorry, but I didn't see if you answered the question about what thermometer you are using. If you are using the one that came with the smoker - those are almost always no good. For my two offsets, 220 reading on the "factory gauge" = 250 real temp!
Also, 99% of the time - ALWAYS KEEP YOUR EXHAUST OR TOP VENT ALL THE WAY OPEN!
You use the side vent, or intake vent (the one on the firebox) to control the airflow.
Stick with it - trial and error - and you'll be fine!
Also, maybe consider starting with something easy. A chicken, or a small pork roast, etc...a brisket is much more towards the difficult side.
Once again, I don't know how long you've been doing this...I'm not trying to be insulting by saying you should try something easy, but there is some truth there. If you start with easier smokes, the enjoyment level is high, the disappointment level is low - and you are more likely to stick with the hobby.
Good luck! This site is the best helper I've ever had while learning the art of real wood-fired BBQ!