Citizens of many countries outside of the EU will require a visitor visa to enter the UK.
You can check whether you need a visa here:
and get information about the application process here:
The earliest you can apply is 3 months before you enter the UK, and the visa fee is 97 GBP. The Home Office estimates that decisions on visa applications are made within 3 weeks, but we recommend that you apply as early as possible within the 3 month window.
Some applicants may need an invitation letter: if that is the case, please contact firstname.lastname@example.org and we will arrange it for you.