Ok, some major things to look at for the guide.

First of all, it doesn't matter what character you play you NEVER completely ignore a skill until lvl 13.

And spikes is actually a very good skill. One point will increase auto attack damage by about 50% to a single target but you also get a cone aoe. This can easily be the difference between killing or being killed. Considering that the damage scaling on scream is not that impressive, you should have atleast one point into spikes.
Personally I get spikes as my second priority skill over scream leaving scream lvl 1 until 14. I do this because spikes are free and allow me to farm effectively using scream only during a burst rotation or to silence someone so rupture will land (no flash ect).
Also, getting 2 strictly defensive items right off will gimp you the rest of the game. Aegis and GA have no offensive component so they aren't going to help u farm.
Personally, I get a catalyst-> Rod of Ages first.Since only two characters have %hp damage abilities (Kog and Amumu)you are safe to stack hp early on in the game. Every time you kill something you regen a % of your hp and a % of your mana. So having large pools of both will give max benefit of your passive. RoA also has the offensive component of more AP and mana giving u more and more powerful spells to farm and pk with.
Personally, I go with the offensive mastery tree as well because of farming reasons.
I've played many defensive Cho's and I beat them all because I farm more effectively which means more gold, more gold better gear, more gear better tank.