All you have to do is add those words to the dictionary, which is as easy as typing them then selecting the actual word you're trying to type from the suggestions area. That should add it to the phone dictionary.
All you have to do if you really want to turn off autocorrect is go to settings, language and keyboard, then hit the settings button to the right of the default keyboard. it's in there, just go to text input and uncheck prediction as well as spell correction and word completion. that should do it. and for the love of god, don't use HTC's keyboard, they make it impossible to put simple things such as exclamation points or question marks, which should appear in the suggestion area when the suggestion area isn't being used for suggestions, but that doesn't even happen.