One thing that might help you get started, is to use the forum post editor to layout your information, then click on the "Source" button (at the top left of the toolbar) so that you can see (and copy) the HTML.

Keeping the formating very simple is probably going to be best for RMP use (the tag restrictions may not be quite as strict as for the forums), but the MO staff should be able to tell you what is or isn't allowed.
Bear in mind that if your item is going to be brokered via CP, the text and markup used will be used against a different style scheme, so be careful if you change colours/fonts from the default.

There are plenty of free HTML editors around (try several to see which you get along with best), including a standalone version of the one used in the forums here.