i cant speak for everyone else, but i do have a LOT of pvp experience (over 100k kills on two different chars) so ill just tell you what i do.
first off, tab targetting. its HORRID compared to FF11's, but its manageable once you get used to it. use that to cycle thru targets, but bear in mind it seems to like to pick targets that are far away first, so if someone is right next to you youre probably better off just clicking it.
secondly, enable the option in your interface menu that keeps left-clicks from detargetting things. i forget its name, but basically it means if you click somewhere else on the screen accidentally (happens a lot when you mouse turn, which we'll go over next) you wont deselect your target. very very handy in crazy fustercluck.
thirdly, mouse turning. i assume youre using the default keys of WASD to move forwards, backwards and turn left/right. in pvp, this doesnt work, because the turning speed is capped when using a keyboard. the best way to overcome this is to use your mouse. hold down the right-click button (assuming default controls) and move your mouse like youre looking around in an FPS (you may want to invert the Y if you think thatll help). while holding the right-click down, your A and D keys become strafe left/right instead of turn left/right. since you should be doing ALL your turning with your mouse in pvp, you can now go ahead and unbind your Q and E keys from strafe and bind the strafe to your A and D. this gives you two extra easily accessable hotkeys to bind to oft-used skills.
mouse turning also lets you keep up with the fast pace of pvp a lot easier, and will be key in preventing people from flanking you and hitting you from behind while also keeping them in front of you (where you can hit them).
finally, for mods, im a fan of xperl unit frames. it allows you to keep track of your party/raid pretty easily (if you so choose) and has lots of neat stuff like target of target (of target of target) monitors that allow you to see who your target is targetting, and even who the target of your target is targetting (thats a bit unnecessary imo, but to each their own).
another key thing to remember is the F and G keys. the default control for F is target assist, which basically targets your targets target. so if your warrior friend is killing a priest, but there are three priests, you can just click on your warrior and press F. voila! youre targetting the same target he is now, and you dont have to cycle thru three different priests to find it. the G key is also quite nice because it retargets your last hostile target. this is especially nice if you lose sight of someome temporarily, or you want to select an ally and then quickly retarget your last enemy target.
most of all, practice. pvp is much more complicated here than it is in FF11, and the only way to get better at it is to do it. duel with friends to get your reaction times down and learn some basic counters to various classes and abilities, and then throw yourself into the pvp environment and learn by doing. of course, feel free to come back here for any advice as we'll try to help you all you can, but most of that will be academic (what counters what class ability and so forth).