There are a lot of things we can do to start preparing. Lots of plyometrics, cleans and variations, foot work drills. You already have a very strong base, and that will carry you a long way. We just have to make sure you have great hip flexibility, and can violently rotate your hips from front to back!
Defensive line is all about having quick feet, hands, and leverage. Depending on what scheme the team runs will dictate responsibilities for you. Generally speaking, you will only ever want to attack half a man, with tight hands.
As far as being a Dend and eventually a LB. Depending on what front you are in, the DEND, could very well be playing LB based on responsibilities. There are lots of unknowns in that part, so lets just work on the athletic part for now as we find out what you will be doing.
For now lets start working in some cleans, Ill try to pull together some videos of basic footwork stuff you can be doing.
As for plyos, I like anything that his hoping and bounding. If you have somewhere to do box jumps, those are great as well.