First of all, create 4 objects:
1. Sprite - slot (invetory slot)
with 3 variables:
id, item, number
It will help us to work with the information.
Add second frame for this sprite to distinguish between the normal and the selected slot:
2. Sprite - item (your items)
with 1 variable:
3. Array - inventory (your inventory array)
Set size to (the planned number of slots (we will use 5),2)
Remember, that arrays in Construct 2 is 0-based. So if you have 5 slots width array, it will start from 0 to 4.
4. Mouse - just mouse object, huh
Add varialbles about which I wrote earlier: