As a dedicated mid, I think I can help you.
Through my experience playing mid almost every game, the most important things are (in order): XP, harrass, turret, kills. Note that this may differ from player to player. As a mage, you need to generally build based on ability power (not sure about Ryze cause I haven't played him). I usually get boots and 3 pots and then 2 Doran's rings, then rush a deathcap for that damage boost. As a ranged mid, your playstyle early game should be to CS and harrass as much as possible, this puts pressure onto your opponent knowing that they have lower health than you. Make sure you conserve your mana and only harrass when youre waiting for a last hit or if they get to close.
As a mid, my playstyle is actually very aggressive mid game if I outlane my opponent. Mids tend to fall late game, so I lower the aggression and just sit back and deal damage while not getting focused. Remember, you dont want to rush in to fights you cant win, always predict the result before you go into a fight and base your decisions off of that.
Know your champ, this is very important and might need expirementing. Know how much damage they deal, what champions you suck/excel against, and the limits of your champ and how you do in certain situations.
Pick a champ based on your playstyle. I usually love to get fed early to mid game and I'm all for damage and no safety. And coincidentially, Ziggs perfectly suites my
playstyle with his high risk/high reward abilities.
So all in all, its about knowing what to do in certain situations. You WILL make mistakes, so try to identify them and immediately fix. Just dont try too hard to not make them, cause thats how you learn.
Hope I helped :)