When you're in a party, all the class icons have little bars underneath them. That's their "hate" meter. Whoever has the fullest bar is the person the mob will attack. Basically all tanking is, is making sure your bar is the highest on all of them.

Okay from my experience of tanking Sastasha only:

As a Gladiator what you will do is use Signs (found in Main Commands) to mark a kill order; usually 1, 2 and 3 - so the DDs know which to attack and don't make your life hell.

You pull and use flash to generate hate on everything, then (assuming you're using default keybindings, as I don't know their actual names) use ability 1 and then ability 3 on the first mob to generate a load of hate on it. Switch through the others and hit them a few times to avoid losing hate to the healer. Then focus on using your ability rotation on the 1st mob. When it's nearly dead you can start building up more hate on the 2nd mob and so on and so forth. You can also use flash every 30 seconds or so.

Be sure to read the tooltips of abilities as you get them to figure out which ones build eminity (hate)