What kinds of things count for capital gains tax?
You generally don't need to worry about selling your personal possessions, unless they're worth £6,000 or more. We're mostly talking about things like jewellery, antiques, valuable artworks and so on here. If you sell something like that for £6,000 or over, you need to work out what profit you made on the sale.
Property is obviously another big capital gains area. This doesn't usually include your main home, unless you use it for business or let it out. You'll also need to tell the taxman about things like sales or shares and business assets.
ISAs, PEPs and things like Premium Bonds and lottery wins are exempt. Also, if you inherit something, you'll probably only have to worry about capital gains tax if you sell it later. If you don't use your car for business, selling that won't count either.
Another odd exemption is for items with a lifespan of under 50 years - although again, that changes if it's something used for business. Of course, your tax-free personal allowance still counts, so if your profits stay under that, you don't have to pay anything.