How to maintain the initiative? What are some ways one can keep the initiative?  I find in many cases I have the upper hand if I am able to keep my opponent defending instead of creating his own attack.  Are there openings or certain moves in the beginning that help keep the initiative?
From my own experience the best way to maintain initiative is remain active. Don't make timid moves, you will occasionally overreach doing this but you will learn what is overly bold and what is simply active play. Also I have found that the better a plan I have the more active my play, it is when I am floundering around not sure where to go from here that I have the worst time keeping initiative. 

Be familiar with the general opening principals, (open lines, knights before bishops, rooks on open ranks, don't make more than 2 pawn moves in the opening, don't move that same piece twice in the opening, etc) none of these are hard fast rules that should never be broken but they will keep you in the right direction in the first part of the game. 

Finally I would be familiar with the basic weapons of chess, (pins, forks, etc) and learn to look for them not only in the current position but in positions that you could reach if you make whatever move you are contemplating. They will tell you both what to watch out for and what tactical opportunities you have to advance your own game, and extend, or update, your plan as the game proceeds. 

One final thought, have a reason, tactical or not but a specific reason, for every move. If you move without a specific reason your are floundering and no longer on the initiative.