For a persons first time I recomand pork butt, it's as simple as it gets and you can learn a lot about your smoker.
You do not have to marinate it, just trim a "little" of the fat off and rub it down with a BBQ rub then smoke it.
I'll trim just a little fat off the exterior of the top half of the fat side down to the layer of lean pork. I always leave the bone in when I smoke pork butts. I then rub the entire butt down with any number of rubs, some I make and some store bought. For the first time I recomand a store bought rub or one you can buy online. Those you can get online are some of the best rubs in the world.
I then fill the bottom of the smoker with charcoal and I add some hickory chips, about 3 the size of my fist. I'll then light a full charcoal chimney and wait until it's ready. I then pour all of the lit charcoal over the unlit charcoal in the bottom of the smoker. Then I;ll put the rest of the smoker together, I'll fill the waterpan with water and sometimes with apple juice. The last thing I put on is the butt on the top rack and cook with temps between 230 and 250 for 6 to 16 hours depending on how big the butt is and how cold it is outside. I turn it over after 4 hours then base it with applejuice every hour from then on.
I also try to watch the temp and keep it up around the temps I posted. You will have to add some charcoal and water to the pan. Make sure you warm the water up first so you don't bring down the temp of the smoker too much.
When I get an internal temp of 165 I'll pull it off the smoker, wrip it in aluminum foil and let it "rest" for 30 mnutes. When you are ready to chop it it will still be hot so wear heavy rubber gloves. Pull the bone out, if it comes out clean and easy it's prefect and done. Chop it all up and you have some great BBQ pork.