One of the best perks on Hogwarts is Here are the lessons they offer to everyone. They're free, and can really help educate you about some things even the smartest witches and wizards haven't gotten a chance to learn yet. One major thing to know is that they are graded like so:

Troll- 0-49%
Dreadful- 50-59%
Poor- 60-69%
Acceptable- 70-79%
Exceeds Expectations- 80-89%
Outstanding- 90-100%

If you get 100% on an assignment, then you earn 10 points for your house, and so on so forth. The questions attached to the assignments can vary in difficulty but are overall easy to do if you pay stout attention! If you want a great score on multiple choice questions, a helpful tip would be to not read the answers. Try and answer the question yourself. If you know it, great! Find your answer in the box, but if you don't no worries, just go back to the text and re-read the part containing the question you have issues with.

Now, not everybody is aiming for a perfect 100% outstanding score, but that doesn't mean they don't want to. For all I know, laziness may be the one trait that could let your whole final down. If you are going to sit down and take a quiz, or write an essay, just do it properly. Your already there, why not spend the time and re-read the text a few times just to make sure? My advice to you would be to never laze off. Guessing doesn't help, and hard work really pays off. Not everyone has to be a Ravenclaw to get good scores.

Just work hard, and stay motivated in order to succeed, simple as that.