VAT in business
If you are a VAT registered business, you must add VAT to your prices. You should make the VAT rate and the amount of VAT charged clear on your invoices or receipts. If you make a sale outside of the EU (e.g. if you are in the UK and your customer is in India), you do not have to charge VAT. Also, if you make a sale into a different EU country (e.g. you are in France and your customer is in Germany) then you can zero-rate the sale provided you have the correct paperwork (see your accountant for details).
A VAT registered business will make regular VAT Returns, usually quarterly (every 3 months), but other arrangements are possible e.g. monthly. Doing your VAT Return involves working out how much VAT you owe your VAT authority, filling out your VAT Return paperwork, and then paying the amount owed.
There is an advantage (well, a silver lining) to being VAT registered though: you can reclaim VAT on your purchases. Also you don't have to pay the VAT and then claim it back. You can deduct this 'purchase VAT' from the payment you make with your VAT Return.
Say you buy an item for £600 which includes £100 VAT, then you sell it for £660 including £110 VAT. You have paid out £100 and you have charged £110 VAT to your customer, so as a business the VAT on this sale has only cost you £10. When it comes to doing your VAT return, you deduct the £100, declare the £10 on your VAT return and pay only the £10 to your VAT authority.