Why don't you split this up in two things:
1) Getting the RSS feed into PHP
2) Saving to the database
The first you can probably do with file_get_contents() on the URL of the RSS feed. This would then provide you with an XML formatted string. By using PHP's native XML support you should be able to access the values in here pretty much right away.
Then you need to define how you want your database structure, so that you can store it the way you want it.
The question you ask is a bit too broad. Answering it would be pretty much the same as writing it for you.
