Owing to the size of the United States, unless you’re coming from a nearby city, your best bet is to fly directly to Dallas. Many international as well as domestic airlines fly to DFW Airport.
You could also ride a bus to Dallas if you’re travelling on a budget or are coming from a city or state that’s within driving distance.
Amtrak trains too stop at Dallas, so you could ride a train as well.
Car rentals are easily available as long as you’re of age and have a valid driver’s license. You could always take a road trip, especially if you’re traveling with a group of friends and interested in some countryside en-route.
HOW TO GET AROUND DALLAS
Dallas has an excellent well connected system of buses and trams that cover the entire city, tourist attractions and all.
The best way to see Dallas would be to take the bus or tram to an area/stop, explore that area on foot and then board the tram/bus to the next stop or back to your hotel.
You could also rent a car and drive around the city but familiarize yourself with the parking norms; the last thing you’d want is a parking ticket.
Taxis are the most convenient way of getting from one point to another, but taxis can be expensive, so if you’re traveling on a budget, you might want to avoid taxis.