Don’t make a customer called “Sales” or “Takings from minicab 1”, etc. A customer should be a real person or company – someone who can owe you money.
To separate out income types, revenue streams, or sources, go to Business Details -> Settings -> Edit Expense and Income Types. The “Transaction Categories and Budgets” box lists expense types you’ve used (and preset ones for your business), together with income types on the second tab. Add your income types there and optionally set monthly targets if you like.