Many countries do not require a visitor visa. If your country does not appear on the following list, you do not require a visitor visa under regular circumstances. See: List of countries requiring a visitor visa
To make sure you do not require a visitor visa, you can use the 'online visa guide' provided by the UK Border Agency (see link below).
How to apply & requirements
The UK offers an excellent online visa guide which we recommend you use to find the visa you require as well as requirements and application steps. It tells you custom information based on your home country and your purpose of visit. See: Online Visa Guide Tool
Certain countries are eligible to apply online, please see the following list to see if your country is eligible. See: List of Eligible Countries for Online Applications
If your country is eligible you can apply online here: UK Visa Online Applications
Alternatively, you can apply at your local UK embassy, but we recommend online application when available to speed up processing time. See: List of UK Embassies