Pork loin is a very good choice for a first smoke...not super-fast, not an all-nighter...gives you some time to let you start getting to know your smoker.
Be aware that OEM smoker thermometers are rarely ever accurate. Add to that, they don't measure grate temperatures, only the space just inside where there are positioned. With that, if you have access to a thermometer probe, it would be beneficial to place that probe near your meat, but not resting on the grate itself. You can stick the probe through a hole in a piece of wood so the tip protrudes out the other side, or through a potato (chef's snack for later).
Hmm, while on the subject of potatoes, you really owe it to yourself to toss some in with your meats. They're easy and delicious...no fuss at all. Everyone I've introduced smoked potatoes to raves about them...same goes for smoked corn on the cob, and squashes, and...well, read my avatar line...
Potatoes cook VERY slowly in the 225* range, but things pick up quite a bit around 250*. 275* makes for a tender potato in quick order. Of course, as with anything, size/weight matters when estimating cooking times. A light squeeze to test tenderness is all they need to indicate when to get them out.
Oh, minimum recommended internal temp for USDA inspected pork is 145*...pretty pink, but I've taken loins and thick chops to upper 150's to lower 160's and they're still very moist. If you don't already have one, a pocket meat thermometer is a good investment...cooking to internal temp ensures safety, and allows you to bring it to the table at your desired level of cooking.