I have long hair, though it’s only slightly below my shoulders at present. I keep it up in a ponytail when in a professional setting, but my preference is to have it braided (especially when I need to be able to keep it out of my way). My wife likes it down, however, so I often leave it down at home. You’ll have to figure out how you are most comfortable, as you won’t look good constantly fidgeting with a hairstyle you dislike.
Above all else, keep it clean! It’s no coincidence that this has been mentioned several times already. Learn about hair care, and figure out what kind of hair you have. Some guys feel this isn’t very “manly,” but is laziness really worth involuntary abstinence? I wash my hair with apple cider vinegar—which is cheap, effective, organic, and has various benefits—and use a silicon-free conditioner. I also use a leave-in conditioner spray to keep my hair under control, but not everyone needs this.