An indoor cat can't sharpen it's claws against a tree, the same way as an outdoor cat can, but it's still a very important part of the cats health and well being. It's in a cats nature to sharpen its claws against something. Unless you want your furniture destroyed, the best thing to do is to buy a scratching post or something else that your cat can use instead of for example the couch. I think it's very important to get a sturdy scratching post, that won't fall over when the cat it playing on it, and also the bigger the better. The one you can see in the pictures above is all I can fit into my apartment right now and it works just fine for for smaller spaces.
Got it from my parents.