Some advices regarding the first research paper I am an undergraduate student in Mathematics and I think that I have discovered something significant in Mathematics. My friends and some professors to whom I have sent my ideas also confirmed its significance. They suggested me to write a paper on it. 

But the problem is that being an undergraduate student in Mathematics, I don't know how to write a paper. Besides, the professors to whom I have sent my ideas weren't experts in this field and they have asked me to send my works to some experts in the field. But unfortunately I don't know anyone such and I see no point in assuming that even if my work is significant, they would give time to read it without dismissing it beforehand as a work of some crank. 

So what should I do? Can some suggestions be provided?
Ask your professor to introduce you to a suitable expert.

If some of your professors think that your work is of substantial quality and probably novel, then they are surely fine with introducing you to some suitable researchers in the field. Note that, for example, a postdoc in the respective field may suffice. They should be willing to give your work the "badge" that in their oppinion, your work seems to have potential by writing that to the suitable researcher themselves. This should get rid of the problem that you describe in your second-last sentence: 

"But unfortunately I don't know anyone such and I see no point in assuming that even if my work is significant, they would give time to read it without dismissing it beforehand as a work of some crank."

Note that an experienced academic in the respective field can add a lot of value to your paper, including improving the accessibility and providing a more comprehensive literature survey.