Some items will fit inside a sims inventory, others wont. If the item you want to put inside your inventory can "fit" a small hand should show up when you hover your mouse cursor over it. Then you have four options:
1-Click the item and select the option "Put in inventory"
2- Click and drag the item to your sim.
3- Click and drag the item to your sim small picture.
4- Click an drag the item to the inventory icon.
5- Open the inventory (press i) then click and drag the item to your inventory directly.
If none of those work the item probably doesnt fit in your inventory. You can move it in build mode tho!