The UK Border Agency has a handy website where you can check your personal details against your need for a visa. You'll need to state the purpose of your visit, nationality and country of residence. In return, it will tell you if you need a tourist/visitor visa for the UK.
The general visitor visa form VAF1A consists of 11 parts. They include: About You, Passport Information, Contact Details, Your Family (including details about your parents and any children), Finances and Employment, Income and Expenditure, Previous Applications and Travel History, Travelers under age of 18 (to fill out if relevant), Your Visit, Additional Information, Declaration and Supporting Documents Checklist.
If you need a visitor visa, you will need to show that you intend to visit the UK for no more than six months and intend to leave at the end of your visit. You must also demonstrate that you have enough money for your trip and to stay in the UK without needing to work or claim from public funds.
In order to prove the latter, you may be asked to provide bank statements or pay slips to prove you have enough money to look after yourself. Your application would also benefit from evidence to prove that you intend to leave the UK at the end of your visit. A letter from your employer should suffice.
If you are visiting family or friends in the UK, you will also need a letter from the person you are visiting (your sponsor), along with a copy of the bio-data page (the page with their photograph) of their UK passport. If they are not a UK national, they must include evidence of their right to live in the country.
If your sponsor is to pay for your trip or support you during your visit, they must also include their own pay slips or bank statements to show they have the money to do so.
It's a good idea to keep any relevant documents in your hand luggage as you'll probably be asked questions by an immigration officer at Passport Control. If any documents are found to be false, you will be refused entry into the UK. Visas are free of charge.
