I just want to be clear that you understand the difference between the basic forms of a link.
- External links are links you make to other sites.
- Internal links are links within your own site.
- Inbound, aka Back links, are links to your site from other sites.
Any link you make to another site (external) should be a high quality link to a high quality site. Meaning not a link go here that takes you to bubba's porn palace. Extreme example, but you get the point. Links should be descriptive and point to sites you trust.
Any link to your site (inbound, back link) should also be of good quality. You cannot always control this. Just be aware of the links to your site and if there are any that are really bad, then ask us here how to address them. We will be glad to guide you.
Any link within your site (internal) should be natural as you would want your users to see them and conversational. Most should be within content or immediately following content though not exclusively.
To put a nail in the point I made before, your link text should give semantic clues with a subject, predicate, and object just like you were taught in your English class. This can be a part of a sentence or a whole sentence and not read more. Obviously, there are exceptions to this rule such as navigation links. There, just use common sense and do not worry about it. All links to other sites should be to sites that you feel are really good and you want your users to know about. Do not make links for the sake of making links. There is no ratio of internal links to external links. There are SEOs that parrot this notion, however, they are dead wrong and you should not take their advice - ever.