8 weeks is very young so don't expect miracles in the toilet training department for a while yet.

The best advice I can give for toilet training is just to be persistant. Take the pup outside regularly, especially just after it has woken up from a nap or had a meal and just wait outside with it.
When the pup poops inside just ignore it, clean up the mess and don't scold. Scolding for going indoors can often lead to the puppy going in more sneaky locations.
When the pups poops outside praise like crazy, make the puppy think it has just done the best thing ever.

Dogs generally respond much better to positive than negative training so praise is the key.

Chewing furniture can be resolved by vigilance and with the help of some commercial products. Pet stores sell bitter spray products that you can spray on items you don't want chewed. Puppy goes to gnaw on a chair but instead of being rewarded with a chew toy, it gets a mouth full of yukky tasting spray that has been applied to it.

Teaching a dog to sit can be easy or hard depending on the individual dog. Again, at 8 weeks don't expect miracles, your pup is only a baby so it may not understand what you are asking of it, but it doesn't hurt to start training now.

To teach a dog to sit the easiest way is to hold a treat above the dogs head so it is looking up. With the way dog are built, when their head goes up, their butt naturall goes down. If the pup doesn't automatically sit down just gently push on its butt with your hand while repeating the command "sit". The second the pups butt touches the ground give the treat and repeat the process.