If you are non- visa national , you do not need to apply for a Standared Visitor Visa before you travel. You can show your travel document to the Immigration Officer at the UK Air port and have the visa stamped to your passport which will be valid for 06 months.
However, if you are from a visa national country then you have to apply for visa as a visit as a tourist, visit on business, visit on study or visit for medical reasons prior to your travel to the UK. You can apply to stay in the UK up to 06 months.