First of all decide on the choice of the land to be used for making of arena. The quality of soil is the most important factor behind the choice of land for the project. Arena should never be built on an artificially filled land. Hire the services of a soil testing company to test the quality of soil before finalizing the deal.
Get a hold of the plat map of the area from the national resources conservation department and ask them the type of soil present underneath the property. Sand content should be high, because it is used in large quantity.
Figure out the level of grading and erosion and sediment in the area. Depending on the local county law, your property may or may not need to be inspected by the county authority for qualitative purposes.
Hire the services of a soil contractor to find out whether you need to add soil to the arena base, and if you need to, then figure out a nearby source of soil.
Hire the services of a contractor to work out on your arena. Remember the most important part is the making of the base; if the base is good, it will lead to a better surface for horses.
Drainage is a key aspect in any arena. You will have to water the arena daily, so it should have a decent drainage system or else it will lead to rising of water table and wet, moist surface all the time. Dig holes in the ground after every 50 feet and connect them with the help of an underground tunnel, so that most water makes its way inside the drain channel.