If you want to be Top contributor, you need to earn a certain number of points through asking or answering.
Relevant and accurate answer can be the rule of getting top contributor. Be sure also that you are always active in answering some questions in Yahoo answers. Speeding the number of your answers can also an advantage.
you need to post quality questions and answers.it will take some time... but you will achieve it if peoples find your answers helpful.
There are certain no. of posting/answers you should complete in order to go with the level 5. It's not that easy to finish in 1 day, but you must have patience and you must love the answering over there, then only you can be a top contributor.
You also need to be chosen as best answer. You can easily make 10 points out of each one so quality answers really are necessary.
For making it to the Top Contributor, you need answer with lots of relevant information and your answer should be pretty long enough. And if you answer continuously, you will earn the badge of top contributor.
Yahoo is a popular site. Its answer is best for back link. But you have to give relevant answer not spam.