You have two routes: spend money on an existing integration, or spend time and build your own integration using the freely available API's.
Here's Salesforce's developer documentation, https://developer.salesforce.com/docs/
And here's QuickBooks Online API documentation,
With the information contained on those two sites, and in a couple hundred hours, you might have a working integration.
(if a couple hundreds hours seems like a lot, our integration, Breadwinner, which connects Salesforce to Xero & QuickBooks Online, has taken a total of 25 developer years so far to build)
If you need further help on StackExchange, you'll need to describe the specific technical problem you're having, otherwise you'll risk your question being "Closed as too broad".