Hi Chris,
Welcome to the forum!
There is a hard way and an easy way.
The hard way - buy yourself some OS 50k:1 maps for the area where you want to ride, look for lanes that are marked up with red "
+ + +" marks or red "
. . ." marks and then work out how to piece them all together. Before you go, get on to your local council and find out which lanes have TROs on them that restrict motorbike access, or get onto Google Maps and look for signposts at the start and end of the lanes. You might also want to look at the Trailwise website (
http://www.trailwise.org.uk), particularly because a lot of ORPAs (the ones with the red dots) are not that interesting. That's a lot of work before you even go riding.
The easy way - get in touch with your local TRF group (
http://www.trf.org.uk/groups/group-listing.html) and get to know them. Most groups will be happy taking you out for a led ride without being a member of the TRF, but after that you should really join up. By talking to the locals, you should find out a lot more than you could research off maps as there is no substitute for local knowledge. You will still need to buy maps, but this way you can limit yourself to buying the most appropriate ones first. Another advantage is you are likely to find riding partners - as a novice, you should never go trail riding on your own. A torn tyre or a bad fall can be a very big problem if you are 5 miles from a main road.
By the way, don't be surprised if people are initially wary about marking up maps. It's a regular occurrence for people to turn up, demanding access to lists of lanes and routes and then disappear without a trace once they have leeched the info.
If you are intent to carry on riding the trails, you really should join the TRF - with your membership you will get access to the members' areas of the forum and the website, enhanced access to Trailwise, as well as helping to fund the Rights of Way specialists who fight to keep the lanes open and adding to the fighting fund which is used to take cases to court. For instance, if you are on a lane that has legitimate rights or where the rights may be in dispute, and someone takes you to court, the TRF will support their members with specialists, not just your local solicitor who happens to be a rambler and doesn't know the difference between a Section 59 and the McDonalds menu.