Well, aside from builds, the like of which there are plenty, there are a few important lessons to learn that should really be experienced rather than just mentioned.
Firstly, you should be aware of extension range, so that you know when you've over-extended yourself from healing, and to be more careful at this time. This is especially true with shadow-stepping.
Secondly, you need to keep an eye on health levels, how fast they are dropping, and from what. Losing health fast from empathy + spoil victor should mean you stop attacking at once, whilst losing the same health at the same speed from other sources isn't quite so drastic.
Thirdly, you need to know how quickly your combo recharges fully - in other words, the time in between finishing a combo, and being able to start a full combo again.
Fourthly, you need to watch your target's actions, and make sure they don't flick on, say, riposte, a moment before you try a long recharge attack skill.
Finally, you need to know what states your build uses, whether it's knock-down, poison, crippled, or whatever, and thier benefits and flaws.
You will certainly need to bring along a snare in PvP, the most common choice of which is Siphon Speed, although Dark/Shadow Prison, and any skill that causes Crippled can also work.
Shadowstepping can be very useful because it allows you to reach your target before they can see you coming, limiting pre-kite and application of stances/enchants/conditions to limit your attack power before it starts.
As for builds, a good starter build is the "Yansassin" (search for it - I can't remember the link). It's pretty flexible, and has a good balance of offense and defence, although in time you will want to put together your own.
In general, the assassin is much more about play than build, even more so than any other class I've tried. Practice, and don't feel dispondant if you don't do amazing first time off.
