I’m a girl, if it matters, but I’m going through the same thing. I’m currently growing out my hair from a short haircut I got a few months ago, and I’m at the awkward middle-length stage. I don’t think there’s a big difference between how girls and guys grow out their hair, but you can disregard this post if you disagree.
I had grown out my hair for my entire life from age 10 to 27, and it was curly and frizzy the whole time. I was quite surprised after the short cut I got, because it ended up being straight and smooth! So, as mentioned above, don’t be surprised if it gets curly or wavy as it grows out.
To cope with the transition, I’m using mousse and headbands. I take a shower in the morning and wash it–no conditioner needed yet. Rub dry with a towel, although as it gets longer I will eventually have to resume avoiding this step (rubbing long hair with a towel is a big no-no, for curly hair at least). Then I put in mousse and slick it all straight back with a boar bristle brush (this ensures maximum smoothness and straightness later on). After it air dries, I brush it all forward to get the mousse residue out–dry mousse will flake, so bend over or else your shoulders will look dandruffy. After that, I just brush it to style it how I like. Sometimes I do a center part, sometimes a side part. Sometimes I brush it all forward and do sidesweeping bangs (kind of how Justin Bieber and that High School Musical kid style theirs). On bad hair days I use a headband–you could just use a ballcap instead. I don’t have nearly as many bad hair days as I did when it was long, though.
Once it gets long enough, you can pull it back into a low ponytail when it’s wet. Try to avoid brushing wet hair too much, especially long wet hair, because that leads to breakage. You can use a wide-toothed comb with rubberized tips to detangle it as needed. If you like to brush it though, get a boar bristle brush because they don’t damage your hair like the cheapy plastic nubby brushes do. My brush is honestly the secret to my style–I can change hairstyles in the middle of the day without wetting or adding product, because the brush is so great.
The nicest long hair I’ve ever seen on a guy was a friend of mine who basically did almost nothing to it. He would wash it in the morning, put it in a ponytail, and go. When he let me brush it, it was sooo luxurious and beautiful. Good genes, of course, but the fact that he never brushed it or heat-styled meant he didn’t have any breakage or split ends to speak of.