You can start before you are 'official'
In many regions you can validate an idea and even take first sales as a sole trader with almost no setup. Registration matters most once revenue is real, you want liability protection, or you need to look credible to suppliers.
Choose a structure
A sole proprietorship is simple and cheap but offers no personal liability shield. A limited company (LLC or Ltd) separates your personal assets from the business and often looks more professional — at the cost of more admin and accounting.
The practical checklist
Register your business name, obtain any required tax identification number, open a dedicated business bank account, and keep personal and business money strictly separate from day one. That single discipline saves enormous pain at tax time.
Get one hour of advice
Rules vary by country and even city. A single paid session with a local accountant early on is cheaper than fixing a structure mistake a year later.