Calibre will do the basics and is probably easier for a beginner than Sigil, which also gets my vote when you want to do difficult things like embedding fonts (which you won't want to do for mobi - it doesn't support them).
One thing, be very careful with your formatting. It's a good idea to nuke your text before throwing it at any program, and especially calibre, to be sure that you've got consistent formatting. Nuking means to convert it to plain text - thus killing all formatting - and then placing it into a new document with the style that you want. A good tip here is to mark special text, such as bold, italic, large font, etc. with symbols like $$, %%, ## so that you can easily find it later to change it back to bold, italic or whatever it was.
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