How can I link to an Asset from the Rich Text editor?
This isn't really related to Craft specifically, but to allow a user to link to an asset, you will need to enable the feature in the redactor configuration file.

For example, within this file:

/craft/config/redactor/Standard.json


You will see something like this:

{
    "buttons": [
    "html",
    "formatting",
    "bold",
    "italic",
    ...


You will need to add file to that list so that it looks like this:

    {
    "buttons": [
    "html",
    "formatting",
    "bold",
    "italic",
    "file"
    ...


Make sure to clear out your cache to make sure that Craft can see your updated file. 

Now when you highlight some text, you can choose "file" and then select an asset from one of your asset sources.