First, what type of antenna are you planning on using?
Second, what are your design parameters (how much gain, directivity, bandwidth, etc)?
As for tools, you'll want to use a software package that solves for the E- and H-fields, so you'll need an electromagnetic (EM) solver. The apps I'm familiar with would be ADS Momentum, HFSS, and CST. All of these are rather expensive packages, and typically purchased by corporations or colleges.
If you want to get started from scratch, you could pick up a copy of Antenna Theory by C.A. Balanis (3rd Edition is the best of the lot). He does a bang-up job of deriving the theoretical equations, and most of the resulting equations are pretty clear to implement a real-world design.
There are other cookbook approaches to antenna design. Many are websites with how-to directions, some are real books. Do a search to find some of them... occasionally you'll happen upon an old DOS program that someone wrote that helps crunch the numbers in the design equations.