The best thing to do is to deep into test automation and automate at least basic scenarios using Selenium/WebDriver tool. But thinking long term you have to invest into making your application testable on all applications layers not just UI. In case you want to check how your application works under high load conditions try to use jMeter tool. In case you need much more details, clarifications or guidance just drop me a call request. P.S. In case you are not ready yet to invest into test automation then hire manual testers on oDesk or uTest or similar freelancers website.

Try to cover basic functionality with acceptance tests at first. There are a lot of tools that doing that. Better to use tools that requires programming skills to write the tests. But if thinking about test automation in long-term is better to start writing test even before implementation. But most of the time that requires mindset switch for all the developers. Test-first, not test-after.