Every Second Life resident has an inventory. Your inventory is where you'll find your outfits, landmarks, props, etc. for using in world. Even new accounts have a substantial number of items in their inventories (look at the outlined number in the image). This number will grow exponentially over years. The account I've been using for building the last eleven years now has over 103,000 elements in it. It's important you keep your inventory organized from the beginning if you want to avoid sorting headaches later.

Usually merchants include notecards and landmarks inside a product's packaging. The notecards may thank you for your purchase or offer instructions about how to use your new purchase, and the landmarks are included so you can more easily return to their stores later. If you buy more than one item from the same merchant, you will end up with numerous duplicate landmarks and/or thank you notes in your inventory. Read, then delete any thank you notes in the package and delete the extra landmark. You only need one landmark, so I recommend deleting the duplicates as they arrive. Merchants will often change store location, so when you are given an updated landmark, delete the old one right away. If you don't, you may forget which of the two is the current one, and then you'll have to use each to figure out which is the correct one. It's a small annoyance but one that is easily avoidable if you get in the habit of keeping your inventory organized from the start.

It's also a good practice to try to remember the names of items you think you'll want to access often. Knowing the name allows you to use the inventory search feature to locate your item, rather than having to scroll through all of it reading the contents.