Well, you're in the right place, but your question is kind of broad. 'How to post a transaction' is really too broad for any forum post. However, eCommerce is the correct tool to use to send transactions into Dynamics GP, I do it all day long.
You'll want to get licensed for the product, download it from Microsoft, then follow the instructions included to install the product. After that, you'll want to write code in VB or C#, you'll instantiate the eConnect object and send it transactions.
You can do that by either passing it correctly formed XML documents that make up a transaction, or by using 'serialization', which is much more complicated but allows more options.
I'm happy to help however you can. Install eConnect and send me your first question.