Check out amazingribs.com for info on basically anything you need including preparing the ribs, brining, rubs, cooking temp and how to know when it's done.
My advice would be to decide they are finished on feel, not on time. 3-2-1 is the 'established' method, (3 hours unwrapped, 2 hours wrapped with some sauce or cider vinegar thrown in, then a final hour unwrapped), but lots of people now say that's too much cooking, especially for baby backs. If you are choosing baby backs I would suggest (at 250F) doing 2 hours unwrapped, 1 hour wrapped then 1 hour unwrapped. For full spare ribs, 2-2-1 or 3-2-1 depending on how much bite you want in the ribs.
Or you can keep things really simple and just give them 4 or 5 hours naked. The good thing about smoking is, every cook is different and there's endless scope for experimentation.
Finally, if you are cooking something for the first time, always give yourself more time than you think you need. Gives you some contingency for ironing our temperature fluctuations, anything going wrong, weather issues, and you can always leave the meat wrapped to rest in foil under some towels to keep in warm if you finish early. Nothing worse than having wives/girlfriends/mates hassling you about when it'll be ready, and not wanting to serve something that isn't yet done.