It's a pretty broad question, and the answer(s) will range from: Free, but you have to know what you're doing to solutions going from US$5000.00 - 10,000.00 (and you have to know what you're doing).
In the simplest form, you may be able to get by with something like NetStumbler (free, www.netstumbler.com). NetStumbler will give you signal levels, channel utilization, and some indication of interference.
Another popular (free) system is Kismet, which runs over Linux. It will also give you an indication of signal strength, channel utilization, and noise/interference. Kismet can also be expanded and use as a form of Wireless Intrusion Detection.
It's great that you are looking to do a site survey, but ultimately, you're gonna need to brush up on the operation, technologies, and parametric requirements to be able to properly interpret the information that the tools provide.
There are no tools that will provide output that will give you explict instructions on how to build your wireless network based on the data collected.
There are some management systems, like the WLSE and the tools provided with the Cisco LWAP that will show you RF coverage, rogues, performance, etc ...in real / near real time. Using these you can adjust a standing system, or, there are even some "auto adjustment" tools in those systems to balance the outputs of your APs for best coverage with the available equipment.
It would help if you could provide some details, then we can steer you in a more definite direction.
FWIW
Scott
