Not advice, and the details depend on where you are. This is a list of topics to make sure you have dealt with or deliberately postponed.
Structure
- Whether to operate as an individual or form a company, and when
- What registrations your country requires before you take payment
- Keeping business and personal money in separate accounts
Tax
- Income or corporate tax where you are based
- Sales tax or VAT on what you sell, including to customers in other countries
- Whether a merchant of record would take the collection and filing off your hands
- Thresholds that trigger a registration
Records
- Invoices that meet your local requirements
- Keeping receipts for expenses
- Bookkeeping, done monthly
Customers
- Terms of service
- A privacy policy that matches what you actually collect
- A refund policy
- Data protection duties if you hold personal data
Protection
- Who owns the code and the brand, especially with co-founders or contractors
- Written agreements with anyone who works on the product
- Whether you need insurance
People
- An accountant, found before you need one
Reply with your country and what you wish you had known earlier. Local detail is the most useful thing you can add.