Fairly simple hopefully, basically I want an item or ability (not sure how items work in SC2 TBH) that, when owned or activated(if it's an ability) adds 10 attack to the unit.

You should get a tutorial on how to do items and everything like that first, but adding damage can be done via behaviors:
create a new behavior -> buff -> go to the behavior tab -> combat -> search for Damage Dealt (Unscaled) and set whatever you like to the value which will be added.

Feel free to PM me if you want me to explain you everything in detail

Items can have behaviors which are set in the item itself.
You can have behaviors for carrying the item and behaviors for equipped items.

For super complex things, you could even use triggers to manage some effects.