DPS and healers shouldn't be running ahead of the tank (though on low-level content they can sometimes get away with playing badly). If people are running ahead of you, there's two basic possibilities:

1) They're new too, and don't know what they're doing.
2) They know they should let you lead, but they don't care.

Try starting out the dungeon by saying something along the lines of: "Hi everyone. I'm new at tanking, so bear with me and give me a moment to mark the mobs and get a blow or two in before you join the fighting. I need to learn how to do this."

The people in group (1) have been given some useful advice about how they should be playing their roles, and they've been given that advice in a context that won't offend their ego. (A lot of people get offended at well-meant advice if they think it sounds like criticism, but by pointing out that this advice comes from the fact that you're new rather than that they are, they're less likely to feel offended at the advice and therefore may have a better chance of following it.) The result could be that you get a better run, and they (as well as you) get better at their classes. win-win

The people in group (2) are likely running ahead because they see this as just another run through simple content, and they may not realize there's anyone in the group who wants more from it than they do (which is just a quick completion reward). They're running ahead because they know the content is simple enough (at least at their level and experience) that they can get away with taking aggro which the tank should properly have. The considerate players amongst this group would likely be willing to slow down to your level if they realize you need them to. They may even have some useful suggestions to make your job easier.

Of course, then there are the players who are simply jerks and are going to get mad at you for being new. These players can fall into either of the 2 groups I mentioned above, but won't take your advice either way. Some will just ignore you and run ahead anyway, while others will berate you for being bad at the game (despite the fact that running ahead of the tank shows them to be the ones who are playing badly). If they actually start harassing you, then you can report them for it, but otherwise there's not much you can do about being stuck with jerks in a random group.

Luckily, despite a few bad apples, the vast majority of players are nice and considerate. Once you get through several runs with considerate fellow players, you'll get enough practice at tanking properly that you'll get both better and quicker at it. As you get more skilled yourself, other players will be less tempted to run ahead of you.