Fitting the correct putter length should be the first thing, and it is a very simple process. Well, it is simple if I am fitting you and we are in the same room. Basically, the player, in this case you, needs to set up as if they were about to putt, gripping the putter as they normally do. Technically, we like to see players with their eyes over the ball, arms hanging freely below their shoulders and in a comfortable and stable posture. Just above where the hands lie on the putter when in the address position, is what the length should be. No magical rulers or measurements, just mark the putter shaft at this point and cut it. If, with your current putter, it is apparent you are going to need to shorten the length, the putter should be cut from the butt end. Important note, you should measure the swing weight (yes, swing weight of the putter) before you cut it shorter so you will have a reference for the head weight / feel. Once you cut a putter shorter, you will loose head feel. In other words, the head will feel lighter. So, most players will add weight back to the head with lead tape, or some method to bring the head weight up, so the swing weight will be similar to what it was before being cut.
Hope this helps.