You may obtain the IDs either by calling GetInventorySlotInfo or from the table below.
The IDs for Inventory Slots are:
0 = ammo
1 = head
2 = neck
3 = shoulder
4 = shirt
5 = chest
6 = belt
7 = legs
8 = feet
9 = wrist
10 = gloves
11 = finger 1
12 = finger 2
13 = trinket 1
14 = trinket 2
15 = back
16 = main hand
17 = off hand
18 = ranged
19 = tabard
Template:WoW API