I will sound like a noob ... but I would like to know... : How do we know if a champion needs Ability power, magic pen etc... or AD and attack speed etc ...
I know what to pick as a tank but my problem is for dps... how to choose between AD and AP... like is it because when you click on the champion, he has more spell power than damage? I don't know ...

Thank you.

bump

Check the AP Ratio's. Do 3/4 or more of their skills have them? Are they good ratio's?

Check the actual skills. Tristana has AP ratio's, but she has Rapid Fire... obvious steroid. Plus her range, and the utility of her skills (a knockback and a jump) mean she's built towards being a physical carry.

In general, any actual carry will have a steroid skill or passive that boosts their damage output, while their skills have utility that mean they could do 0 damage and still be useful.

Short answer is to look at their skills and see where additional damage will come from ( if there can be any ) to find out what's most benefitial to get.

Blue numbers mean that addition damage will come from AP
Red comes from AD.

If your character has some "# of attacks" based ability (like yi, xin, or TF) then it's a good idea to focus a bit more on AS

And in general, APen wins out over pure damage, and MPen wins out over AP but you only need runes, and maybe one item with armor penetration since the APen items are better..

also, if you're not sure how to build a champ, go look up some guides. There are some on the forum here and also some good ones on http://leaguecraft.com/strategies/guides

An Abyssal Scepter enhance (magic)AOE-damage champs like Amumu that are in the middle of the enemy. And also providing important MR.

Your items should pronounce the strong sides of your champion and even better, if they are also ease the disadvantages they might have.