There are many things to consider when going for your first site, obviously the first thing you need to do is find somewhere to have it, when looking for this you need to consider.
1. where are the nearest houses.
2. access
3. land classification i.e if woodland is is ancient? or a county wildlife site you can check this on
http://www.magic.gov.uk/
how many people will you cater for, if public allow for approximately £200 per person for kit
4. public or tourneys
There are just too many things to list but I would be happy to help if you needed any.
I own two sites so i've been through the hard bits.
Drop me a pm if you need to know more.
oh, and the 150k that someone mentioned above take away 149k and thats how much it initially cost me upfront to get my first site started (not including markers) then i just evolved my site on the turnover.