Galio is a nice tank, and if youbuild him correctly he can be a really high dmg dealing tank because of his Passive:).

The items you have to buy are a Dorans Ring;
Next you have to buy boots for CD reduction;
Next is a negatron cloak and evolve it into a banshee;
After that you must buy some armor (if there arent any AD champions on the enemy team skip this item), start with a Giants belt and evolve it into a sunfire cape later:);
Next buy a Guardian Angel, you know why:).

So if you play Galio you have to farm, and preserve your mana. Harras with your Q, get the blue buff every time you can, at level 9 or 10 you should be able to clear the minion wave with your E and Q fire your E first and then your Q to the GROUP of minions that have the least health.
Finally when yo use your Ultimate Idol of Durand, Use it to initiate team fights and get between 3 to 5 enemies and go for it :), i reccomend to save flash to get between enemies, but NEVER use your ulti on 1 enemy, use it on 2 enemies to hold them off so that an ally on low HP can escape.

The masteries are 1/10/9 :).

The runes are per lvl armor/ magic resist and mana regen because Galio uses a lot of mana :)

I take Ignite and Flash for summoners. Flash is good for escaping and for getting betwwen enemies for the use of the ultimate:).

For farming use ur E and then your Q when u have reached around level 7,until then, last hit :).