We were all in your shoes before, and I can start off by saying, DONT EXCESSIVELY BUY until you organize your inventory and until you know that you'd commit to playing SL! Literally, it will save you in the long run before you start purchasing items. I organize specifically by Accessories all the way down to shoes. Having these folders will help you tremendously.
If you're trying to put the items on your avatar, which you unbox (if you haven't already done so) and accept the items to put in your inventory. Most cases, most items will have you do that, and you'll get a notification in your top right corner to accept them. Now to apply, you right click on the items and click "Add" otherwise if you double click, you may notice that parts of your body start to disappear. It would be nice if SL could edit that, as double clicking is quite easier rather than right clicking to add the items to your body. However, Make sure they fit first, and you can identify key words like *Maitreya, Slink, etc. located on the specified product before trying it on. Otherwise, If you see that it looks odd, then it is not for your avatar.
Good Luck!