Please Check if you need a UK visa via https://www.gov.uk/check-uk-visa
If you have to apply for a Standard Visitor visa
You must apply for a Standard Visitor visa online before you travel to the UK.
A Standard Visitor visa costs £115 for up to 6 months. The earliest you can apply is 3 months before you travel.
If you visit the UK regularly, you can choose to apply for a long-term Standard Visitor visa instead.
More inforamtion: https://www.gov.uk/standard-visitor.