OK cool, that gives me enough information to give you some direction, at least to get started.
Keep in mind I tend to take a very practical approach to things since I am terrible at maths... so I'm sure there are better ways of going about this, but, this should work just as good.
Lets assume you are making an inventory that can hold 5 things, and it's a menu/quick bar somewhere on the screen.
The first thing you will need is a global value to indicate the amount of free spots in the inventory, so in this case 5. Make a global value called "inventory" or something like that, and at the start of the layout set the number to 5, so you have 5 free spaces.
Next, make each square of your inventory as a sprite object. You should be able to make just one object and copy+paste the rest. Each of these objects will need a private variable called something like "item", and the values will need to be set to 0 (0 for no item, 1 for item is in that spot). Again at the start of the layout set all the private variables for each box to 0.
Now, you will obviously need some icons for your items so you can see them in the inventory when you pick them up etc. Once you have these, and have some items to collect...
When you collect an item, compare the global value "inventory" and make sure that it is at least 1 or greater. If it is 1 or greater, then minus 1 from that value, then pick one of the inventory boxes, and compare the private value "item" and make sure that it is equal to 0. Make that value 1, then set the icon for the item type to the x.y of the inventory object.
It is a very basic way to make an inventory system, but it should work pretty well, especially since you don't seem to require any crazy dynamic inventory sorting system.
Basically see if that information helps... if not then I'll try to make something simple to give you the idea, but I haven't got a lot of time lately since I have been really busy at work etc. Christams is coming and we are starting to pick up in the shop already.
Hope that helps a little at least.