If you're looking for relatively simple attribute changes, TF2Items as Drixevel mentioned would be one way to go about it. There's also
Rebalanced Fortress 2.
For creating new item behaviors, you'll practically have to learn SourcePawn, the scripting language used by SourceMod.
I've created a bunch of custom "attributes" as SourceMod plugins; many of them are available
here; most of these require a lot of familiarity with SourceMod's tooling, so I can't recommend using them as a beginner's reference, but you can see what's possible.