How do I add the starting balance of my business
That depends on what you mean by “starting balance.” Read these Guides:
Starting balance of bank account
A bank account is a cash account. Read the first guide above. Before any starting balance can be entered, you must set a start date under Settings. You do this only if you are migrating to Manager from another accounting system with an ongoing business. Otherwise, first contributions to a business account use a Receive money transaction, posted to an appropriate account in the Equity group.