Practice practice practice.
Also, keep in mind that a lot of what makes good tanks very good comes down to leadership. Learn the places you'll tank like the back of your hand, every pull. Set up marks before every pull, and do it based on the needs of the pull, not randomly. Once you're strong as a tank, you might be able to use the marks less in certain situations, but you should never stop using them entirely, because they are a large part of what brings cohesion to a group. Know how the boss fights all work, and think after every fight what you could have done differently to make the fight easier for the dps and healer. Your job as a tank is to make the run easy and smooth for everyone else in the dungeon. Positioning, cooldown usage, debuffing, interrupts, and verbal directions all add up to make the run easier on the others. Always be looking for a way to improve, and never scoff at others giving you advice. If you're sure they don't know what they're talking about or it's something you know already, just thank them for their input and move on.
Do this over and over until tanking is something you could do in your sleep, then start looking at apping to a guild, or running PuG and semi-PuG raids. If you're a good tank, you'll get noticed, and eventually someone will ask you to join them. When that happens, make sure you know the fights in the raids every bit as well as you know the 5 mans you've been tanking, and think about the fights from the angle of a dps and a healer as well. You can get a lot of info for those by watching vids of the bossfights and reading the associated threads.
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America