As FB says, it's not a long route but it's as well not to overstretch yourself if it's your first time wildcamping on Dartmoor.
The walk through Fernworthy forest is pretty boring but once you get out of the trees, the view opens up and the valley is beautiful, with the North Teign running through it. Teignhead is a great spot to camp - as you enter the farm from the North-east, there is a grassy, gently sloping spot on the left that is ideal for a tent. Plenty of water nearby in Manga brook.
Car parking is a bit problematic. Don't use the visitors' or fishermen's car parks as overnight parking is specifically prohibited in those. If you head to the western end of the reservoir, you can park there (no charge) but you have to accept the small risk of leaving your vehicle overnight.
Let us know how you get on.