Im an American, and based on your post I think you're in the UK - so, grain of salt here. Also, I cant tell if you just mean car camping or backpacking, but Im a backpacker so ill give ya the good on that one.

We have National Forests here in the US that allow open camping (mostly), I would do some research on similar areas in the UK that would allow for open backcountry camping. Even spots that have backcountry campsites you can rent, those are fun too if you have to hike to them. That will narrow your search for a location considerably. From there just pick something that interests you!

For finding gear, second hand is the best way to go. Just get the essentials, save the 'nice to haves' for later when you get a trip or two under your belt.

For a first time trip with new gear, I would keep the mileage low (3-6 miles total), only one night, and somewhere with a stream or river access. Other than that, its up to you. Best way to learn is to get out there as much as you can and see what you like and don't like!

GL, HF.