Learning to Code
 

If you are plaing to learn how to code you need to learn it in steps.
First you need to go and mast HTML becasue it is very important.
Then once you are good at HTML move up to php and MASTER it. Once you have mastered php you can move on to whatever kind of coding you want such as Java or whatever you are intrested in.

How much HTML exactly? Basic tags? More advanced?

Imo, beyond mastering HTML, you have to get into the habit it of incorporating it with CSS and tableless to create 100% valid pages. Then, you can go off in whatever direction you want to go, because beyond XHTML/CSS, it's mostly personal choice.