Its quite an easy process.
You have to go to catalog - add products and then click on " am adding a product not sold on amazon"
Then you choose your product category.
Once you arrive at the page. Fill in all information.
While filling in the information, you would realise that you need certain information ready.
Like the dimensions of product, description, SKU number etc. Some information can be skipped, but some required and will be highlighted in red. When u finish everything you submit and it may take hour even a day.
But take your first listing as a learning process. You can always delete the listing later and create a new one, until you are satisfied.

There is also another method of creating listing using xls. But i suggest don’t do it. Its bit complicated for begineers. But you can always download the xls and see the information required for future listing.

ASIN number is automatically given by Amazon after you complete the listing. What you need ready is a SKU number. You can make up your own SKU number ( by using tips on internet )
Another important thing is to get your brand registered on amazon, even an application copy will be accepted for starts.

All other stuff you will learn while using Amazon, it is better to do it all youself without any professional help. Lot of times, even amazon reps can help you out.