The most commonly tested errors on the GMAT are those involving verbs (subject/verb agreement, tense, consistency), pronouns (ambiguity, agreement in number), placement of modifying clauses (ambiguity, wordiness), and parallel structure (items in a list, comparisons, transition words). Study accordingly to make the best use of your time. Spelling is not tested.
The first answer choice 'A' is always the original phrasing, ie it's the answer choice that does not make any changes. Of the 5 choices A, B, C, D, E, answer choice 'A' is actually the correct choice about 1/5 of the time! So, don't be afraid to choose it if you think the sentence is correct as it is. A good GMAT rule is don't overcorrect.
If you're stuck between two choices and they both seem grammatical and unambiguous, pick the one that is shorter or simpler. Sheer wordiness is considered an error in some sentence correction questions.
Practice doing these questions under timed conditions. Ideally, you should be able to do them in under 30 seconds. Doing them quickly will buy you more time for the reading comprehension and critical reasoning questions. Good luck studying.