Alright, widgets made simple:
The general idea of widgets is to replace your sidebar content with the widgets.
Sidebars generally consist of blocks of stuff with content in them. In widget terms, each of these blocks is a widget. You can arrange and rearrange these blocks on the widgets control panel (Presentation->Sidebar Widgets).
So for any given block of sidebar, you’ll want a new widget. If the widgets you have don’t fit what you want, you either need a new widget which does fit, or you need to create a new widget/modify an existing one.
Instead of editing sidebar.php for anything, you go to the widgets control panel and mess around with the particular widget you want to mess around with.
To add new widgets (like those at https://widgets.wordpress.com ), you download that particular widget, upload it to the plugins, and activate it. It’s just a plugin like any other. Once you’ve activated it, the new widget will be available to you on the widgets control panel, and you can drag/drop it, configure it, whatever.
In other words, to mess with the sidebar content, generally you’ll go to Presentation->Sidebar Widgets and mess with it there. Nowhere else (hopefully).
Now, about your “Credits”. Probably you want to create a Text widget. So you’d go to the widgets control panel, drag a Text widget up to the sidebar, then click it’s configure button. You’ll name it “Credits” and then put in any HTML you like for it. Then you save. And voila, you have a Credits box in your sidebar.
The idea of widgets is to eliminate the need to ever touch the PHP code for your sidebars. However, there’s not a widget for every occasion (yet), so sometimes you may need a custom one here or there. If you do need that, then we can help you either find one or tell you how to create one.