Happy to help out. If you'd like to track capital contributions or investments in QuickBooks Online, you'll need to setup an equity account. I've outlined this process for you below. You'll be a pro in no time.
Go to Settings ⚙, then Chart of Accounts.
Hit New.
From the Account Type drop-down, choose Equity.
In the Detail Type drop-down, pick Owner's Equity or Partner's Equity depending on your situation.
You can follow the same steps provided by @Michael K above. I also suggest consulting your accountant or bookkeeper on how to proceed. They can ensure that the account created works best for you and your business.