Those of us that get asked, and I'm sure you'll know who answers a lot of the questions round here, sort of aquired the knowledge they have. Thing is if you asked them to tell you what they know, they probably wouldn't know where to start, and neither would I.
Your best bet is buy a good manual and start by doing your own servicing. Get to know what's what on your truck and by staying on top of it you'll soon know when something isn't right, we call it preventative maintenance. As you confidnece and tool kit grows don't feel abashed to keep asking advice, there are plenty of people on here glad to offer advice if not muck in and help. Learn by doing :)