How Can I use A1 Sitemap Generator???

Get started by reading the tutorials

Here's a "sticky" topic placed at top in A1 Sitemap Generator support forum:
Sitemap generator help and tutorials

It looks complicated. Isn't there any simple step to use this.

Here's the tutorial for creating XML sitemaps specificly
(linked at other page as well, but maybe you did not see it?)
http://www.microsystools.com/products/sitemap-generator/help/xml-sitemap-generator-tutorial/

It really just is to type in your website address and hit the "start scan" button
If you want to create a different type of sitemap, just check the relevant tutorial.

If you have advanced needs, you can switch off "Easy mode" in A1 Sitemap Generator which will give you access to the full range of options. But for the majority of users that should not be necessary.

If you need help with anything, please feel free to ask

This is the first time I am hearing about A1 sitemap generator, I will take a try now.