Wear and tear and warranty

No warranty for these items:

• pads wear. If you use them in wet and sandy conditions they wear faster.
• front wheel damage is your fault
• chain grows and wears, especially if you don't shift properly
• cassette wears, especially if you don't shift properly

Having said that, I don't think that you need to change your chain and cassette. Unless you do racing you can get away with the same chain and cassette for 2-3 years.

Many things on bicycles are consumable and anything that moves or rubs is going to have to be replaced, it's just a matter of when.

It's unusual for a cassette to wear out completely within a year although if it's been ridden on a stretched chain or without regular cleaning and lubing it'll wear out much faster.

In my experience chains are normally the first things to wear out, presumably because they almost constantly in motion and rubbing against the cogs while you ride plus picking up all kinds of stuff that flies off of the wheels. I normally replace my chains every year regardless of the state that they are in simply because of the low cost of a replacement chain and the relatively high amount of damage that running a worn chain can cause.

You can tell if the cassette (or the chainrings) need replacing by checking the teeth. If the teeth looked ramped you should switch out the part. See Image:

If the teeth don't look worn but are black with dirt you can clean them with a de-greaser you may find it easier to clean the cassette if you remove it although this requires a cassette removal tool otherwise you can leave it on and clean it with a brush and ordinary dish soap, just rinse afterwards, dry it and remember to lube the inside of the chain again after cleaning it being careful not to overdo it as that will just cause muck and grit to stick to it while riding.

To check the wear on your brake pads look at them from the side and check for grooves. New brake pads come with groves which allow water to escape from under them much like the tread on a tyre. If your brake pad has no visible groves on the face that touches the rim, they should be replaced.

Chain wear is objectively measured. Ask the shop to show you the chain wear measurement. It's a simple device that shows how much the chain has stretched. But if the chain is really worn with such small mileage, then something else may be wrong. Even if it has never been cleaned or lubed, I doubt that it could be worn out already.