This is only a little guide to show u some usefull tricks for playing Gragas
Let´s start.

For Runes I use the standart flat ap runes for my AP mid.

Normaly i go 21/0/9 but if u laning against a heavy early burster like Le Blanc u can Change ur Set up and go 21/9/0 (get the MR and extra life).

Start whit boots and 3 health potts. Try to get as many farm as u can. Next time u are back purchase 2 dorans and maybe a few healt potts and a ward. Now the only thing u do is farm, farm and farm dont be to greddy :D i know that. When u are back another time get the lvl2 boots and maybe the catalysator for ur RoA.

First u skill ur q. Thats ur main Farm ability. The secound skill u should lvl up is drunken rage -> gives u some nice sustain whit the mana reg and dmg outputt.

Ur main farming combo at the beginning is to throw ur barrel between the caster minnions and the melee minnions. Autoattack the melee creeps and bodyslam the casters.