I'm running the 8.1 update, so I don't know if this is true in 8.0 (but I think it is), but just type and then highlight the word you want to add, and one of the autosuggest words will have your word with a + in front of it. Hit that suggested word, and you'll add the word to your dictionary. I haven't found out how to delete or edit a word in the phone's dictionary. You can clear all the custom words in the dictionary. I haven't found this anywhere in Microsoft's documentation, which is frustrating. I found it at allaboutwindowsphones.com, which is definitely going in my favorites alongside wpcentral.com!
Chris