Welcome to the boards!
As Unknown said, the best way to learn the tech tree, resource costs, and map layouts is to play a LAN game with a few bots.
When you're actually in-game though, commanding is *tough.* You won't find many good commanders, because it's a tough role to play. No one wants a new player commanding them, so if you jump in and start messing around, you will most likely be ejected.
If you join a server and your team is pretty well-secured and you don't stand a chance of losing, just tell the current commander you want to learn and ask if you can jump in for a few minutes. They shouldn't really have much of a problem with it.
On the other hand, try jumping in when your team doesn't really stand a chance - it'll give you practice prioritizing things when you've got 6 guys screaming for health and ammo.
I'm a pretty good commander (or that's what the rest of the people in-game say, anyway), and I attribute it to having played RTS games for the last 9 years.