Turn your word processor software's spelling and grammar checker on [go to Tools, then put a check next to Spelling and/or Grammar]. This useful tool will identify [usually via colored zigzag underlining] words not listed in the software's dictionary as well as a variety of grammatical errors such as subject/verb agreement errors. However, it is only helpful if you set it and use it correctly and if you have faith that what it tells you is correct. Learn how to set and use it. Use it, and then pay attention to it it when it tells you something is wrong.

Most grammar checking software can be set to identify everything from subject/verb agreement errors and use of passive voice to how the number of spaces between sentences.  Look for Options and Settings in the Spelling/Grammar menu.  Check the boxes for errors that you typically make and uncheck those that you don't make or don't understand.  The less you check, the more meaningful the underlining becomes.  If every sentence is completely underlined, you won't know what to fix.   

Also, for those cases where you used a spell checker but your professor found spelling errors, know that the same errors will recur in your writing and make you look uneducated until you learn to fix them. Learn to avoid these errors by keeping a list of the ones you make in other assignments and drilling yourself regularly over the correct spelling.  You may also add words to your spellchecker's dictionary [select Add].  Books of "spelling demons" may also be of assistance for persistent spelling problems.