I think tf2lobby is a good intro to the world of competition, really. It works well for getting you the basic training through example. That said, unlike the OP, I certainly have experienced a number of matches where there are really terrible elitist players that make the game no fun; the amount of flak you'll take for being new to it really depends heavily upon which class you play.
If you're a scout, not many people are going to mind if you don't know your way around; the demo is mostly the same way. I notice that it's the soldiers and the medics that take the most flak for being 'terrible'. Pocket Soldiers need to not jump away and leave their medic defenseless (which, since I play medic, I tend to notice a lot), and I've played with a number who do and don't tend to realize it. Plus, if you're the medic, you need to know who to give healing preference to, and I guarantee that if you're playing with an experienced team, everyone will have different opinions about that order of healing, and some will be vocal about it.
But: long story short, I agree heartily with the OP. go out and do it!