For links in text widgets see here > http://en.support.wordpress.com/links/text-widget-links/
To create links on page or post see here > http://en.support.wordpress.com/links/
Copying and pasting can lead to trouble unless you do so correctly. Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Now, starting with your latest post, open it in the editor, make one minor change such as adding a space and then deleting it, and then click “Update Post.”
If you have been copying and pasting text from elsewhere without using either icon 5 or icon 6 in Row 2 of the visual editor, please see here so you know how to do this correctly in the future. http://en.support.wordpress.com/visual-editor/#pasting-text