The quickest way to try out the API is to set up your demo company and dive into the API Previewer. Most of the API functionality is supported and you can quickly start playing with real calls against demo data.
We've got SDKs to cover the most used technologies in the community but we'll never cater for everyone. If we don't support your particular tech then your best bet is to look for help on our developer forums. You can also use our OpenAPI spec to generate your own SDK
Hopefully everything you need to know is on developer.xero.com but if you're still stuck then you can reach out to other developers on community or stack overflow. If you're really stuck you can even hire Xero certified developer to help you out.
That's really up to you! Get connected with accountants and business owners to find out how you can help them be successful. There are plenty of resources like our business forums, developer forums and UserVoice page to get you started with some ideas.