Well, I don't think many people will help for for a whole load of reasons. Before using roids, you really need to be pretty much as your full natural potential until going to the next level. OK, very few people are that patient, but you are asking the wrong questions to get responses.
1) Firstly, you are only 20, probably a little young to start using the juice.
2) You don't state what your goals are - By this I mean that you say you want to get 'fitter'. Running will get you fit, not steroids
3) I wouldn't recommend running deca without stacking it with
Test
4) You ask about gym work and diet - BOTH of which you should already have sorted, not just starting to do it now as your will try to get the most out the cycle
I don't mean to flame you, but these questions show you don't know how to train or eat right, so whats the point in taking the deca. Read the training section, buy a book, see how your body reacts to training and diet, then if you really have to, repost your question and people will help.