Joshua Tree can be a little wierd with setting up top ropes. Often the scramble to the top of a crag can be more dangerous than actually just leading the climb if it's well protected and in your climbing range. As others have mentioned, make sure you know how to set up a natural pro anchor before you die. Assuming you do that, some technical tricks for setting up a TR that are unique to Josh that I have learned the hard way...
1. Take a lot of webbing and/or belay from the top of the climb. More often than not, the top of a climb is a gradual lessening in steepness, and anchor cracks are far back from the top of the climb. Scraping your rope over fifty feet of almost horizontal rock surface is not a smart thing to do. I've seen at Indian Cove that the webbing from the anchor to the TR was longer than the rope itself.
2. Take some big cams and practice using them in flaring cracks. I can't count the times I've led a thin crack or bolted face, only to confront some 4 or 5 inch wide flaring crack at the top as the only gear placement option. Take your time placing your cams in these grainy nasty things to make sure your cams don't walk or tip out.
3. As others have said, learning to set up TR anchors on a forum is not wise, but specific to Josh, I ALWAYS place 4 solid pieces in as many different configurations and cracks as possible, and recheck it after it has seen a couple of uses. Another trick for TR's is to utilize some of the enormous boulders with a section of static line or a long length of webbing. I'm not talking tippy little rocks here, I'm using boulders that are bigger than a couch.
Ok, some areas that I would hit up:
For bolted anchors (hey, this is just like the gym!)
try Echo Rock. You will need two ropes, or (better) belay from the top. There are some 5.10 face climbs there that are great for a burn, and you can walk around the back right to the TR bolts. Also, lead something on Headstone, the bolts are closely spaced, and there's a fat bolt anchor on top.
For natural areas: Trashcan is always popular, and overrun, I saw I guy fall from the top while setting up a top-rope, he lived but it killed my Mojo for the day... Atlantis Wall has pretty easy set ups, and there is an actual edge to rappel over, and have the traditional top-rope set-up where the belayer is on the ground. Belle Tank is out of the way, and has some good climbs in both sun and shade, with an easy and fun scramble-up to set up anchors. Echo Cove has some good routes, but the scramble up can be tricky if you are new to the game. Indian Cove is also overrun with top-ropers belaying from the back of their SUV's. Thin Wall is Ok in Hidden Valley, may have to wait in line, but the routes are fun, and TR'ing is fairly easy.
Gearlist: Enough biners and lockers to make everything work: 6 biners and 4 lockers is a minimum.
50 feet of webbing or cord, plus a few single and double slings.
6 cams with an emphasis on larger cams: My usual routine is two extended range cams (I like Trango) with a 1,2,3,4 BD Camalot sizes, doubling the 3 if I know it's a fat crack up there.
1 set of stoppers or nuts. Oh, and a nut-tool.
A fat rope: Those skinny ultralight sport climbing ropes get trashed quickly here. Get a big heavy 10.5 for all that rasping and scraping it will get, and for God's sake, NEVER let it sit in the dirt\sand, that shit is sharper than you think.
You will really limit your TR possiblities if you have less than that.
Ok, I hope I havn't insulted you with some of my suggestions, but since you look pretty new to the game, and since I've made every mistake you can without getting killed at Josh, thought I'd try to steer you in the right direction.