Great question! First, check your visa conditions — most Skilled Worker visas allow self-employment or directorship of your own company, but verify this with your visa letter or on the UKVI website.
To register a limited company (recommended for a restaurant), go to Companies House (gov.uk/register-a-company) — it takes about 24 hours and costs £12 online. You'll then need to register with HMRC for Corporation Tax within 3 months of starting to trade.
For a restaurant specifically, you also need to: register your food business with your local council (free, at least 28 days before opening), get a food hygiene rating, and check if your premises needs a premises licence for alcohol.
The Nepalese Business Directory has a good list of Nepali-speaking accountants and solicitors who can guide you through the whole process. Worth the investment!