You can download WebLOAD for free at: http://www.radview.com/download/download.asp?Pgm=1
I would suggest that you look at as many tools as possible and try to find one that suits your environment.
Obviously there are applications which you can download and test yourself but remember that most vendors will also perform on-site demos running against your app's and in your environment.
For the actual test itself, I would suggest you identify the 'business processes' or 'user transactions' that you wish to incorporate into your test and then attemtp to establish roughly how long each transaction should take (This kind of info is often best supplied by your business analyst).
You then have the basis for your test and some 'goals' with regards to anticipated transaction times. e.g. If your userload passes the anticipated transaction time within the expected time frame then this could be regarded as a successfull test
One thing to bear in mind is transaction pacing. If you let your users run flat out with no pacing between steps/transactions I would call this more of a 'stress' test, however if you pace your transactions out this is generally more representative of actual user interaction and generally provides more meaningful results.
The only free tools I have tried are Microsofts WAST and OpenSTA (which I would certainly recommend taking a look at).