The easiest way to do that would be to just go to Security -> Manage Users on the main menu. You can add, delete, and update users there (to update, right-click on a user). You can use an unused field in the User Profile (e.g. zip) or an "extended" field to store their credits (the first one would be faster and easier).
You might look at the Batcher extra (though it's a little out of date for the current version of MODX). It could be modified to do what you want.
Another (more up-to-date) option would be the CMP that gets installed with the Example project in the MyComponent extra.
Both are for resources and elements, but could be modified to work with users instead.
If you need to put the User data in a custom table, take a look at the ClassExtender extra and see this: http://bobsguides.com/blog.html/2014/05/27/why-extend-moduser/
This is good but not as my requirements. Actually, I need a way to add my custom pages in modx manager and those page will be accessable via menu like in menu Ii will have a link of ADD Credit. When I click on Add Credit it will show a page with heading Add Credit and some text fields and text area and submit button. So for this purpose I think I should need some templates and controllers as I visited some blogs and tutorials, but I didn't get any information and page linking. I just want to know that how I will create this custom page with my own coding and style in manager side.
I think this might help you: http://bobsguides.com/blog.html/2014/03/28/modx-2.2-cmps-an-anatomy-lesson/
The action class file designates a 'template' file. The contents of that file will be displayed in the Manager's right panel. It can be full of JS and Ajax, but it could also be a relatively simple HTML form.