I currently send email to subscribers in a plain text format.

I wish to start sending email in html to make the emails a bit more attractive.

Is it just like writing an html page, is there certain things you can't do when sending html in email?

I know this is kind of a broad topic, a link to a tutorial would be awesome.

Thank you in advance for all replies.

is there certain things you can't do when sending html in email?

To an extent yes. You might want to check out www.campaignmonitor.com. They have tons of free templates, they handle the mailing list for you, and the cost to send out newsletters is very reasonable, like $5/100.