There are quite a number of taxi companies in Dallas and in and around the downtown are you will see quite a rainbow of them. The most predominant one would be the Yellow Cab but there seems to be no shortage in the downtown area or the airport. Anywhere else, you will probably need to phone ahead for one unless you are at a hotel.
you need a car with working A/C, Renta cars are sometimes poorly maintained, make a point of bringing this up (politely) while renting the car to make sure they give you one that has recently had maintence.
If noone in your group drives: cabs are unrelyable, you may want to look into hireing a car with a driver, depends on how precious your time is to you, have waited up to 3 hours for a cab before, Texans are not really cab people, and we just don't have that many cabs around, if there is a big convention in town then you may just have to wait, but sometimes that is the best part... West End Cab is the best company that I know of but they don't like to get too far from downtown, I think their working peramiter is no farther north than Love Field. If you are Driving, Spring for a MAPSCO. It lays the whole city out on a grid and has an index where you can look up any address and find its grid location. Whoever sells it to you should be able to explain how to use it better. It will run you about $30.00 and is worth every penny.
ADDENDUM - Yellow Cab ate West End Cab and have gotten much better, otherwise I recomend Ranger Cab. I've been having to bop around downtown a lot lately and have been using our cities lovely cab service, and I wish to tell you that it is vitally important to use a large name brand cab company. Some of those guys have been really, really scarry.
I think this picture speaks for itself....
Ok, this turnpike was just built recently for George Bush!
Will there be one for George W. Bush, if there is you will understand why the traffic can be confusing in Dallas.
Highway 35 West goes north, south, east and west. Whew!!
If you're coming from downtown Dallas you can take Main, Commerce or Elm Street which will take you into Deep Ellum. The pedestrian area begins at Good Latimer which is just past Central Expressway.
Arriving from I-75 (Central Expressway), you can take the Live Oak exit and continue on until you reach Main or Commerce (driving under Central Expressway.)
All depends on where you're coming from. I-35 will get you there from anywhere almost. I-20 will get you there from down south. Check out your atlas and plot your route accordingly.
By bus, rental car, or find a friend that is willing to drive with you to check out the sites. But don't go trusting strangers right off the bat. Ya might not wind up coming back.