You can start by checking the relevant visa route and current application guidance on GOV.UK, including eligibility, required documents and fees. For help with a complex case, consider speaking to an immigration solicitor regulated by the SRA or an adviser regulated by the Immigration Advice Authority (IAA). Be cautious of anyone promising a visa or asking you to submit inaccurate information, as this could harm your application. The final decision is made by UK Visas and Immigration based on your circumstances and evidence.