Pay attention to the tense, noun ,adjective,verb,wrong spellings...
There is always 1 mistakes in one sentence,so do not think that there are two mistakes.
Don't be frightened of proofreading. Practice can lead you to success. I can recommand you to buy the following proofreading supplmentary exercises:
Effective Proofreading Tests
Goodman Publisher
Dr. M.C. Cheng