I want to know how to make fireball spell and do not know how. And i research for method yet don't know in detail. So i go for here and ask then i got on hold for my question. Why? This is my question. because i want to do it with blender and unity, is it the reason i can't ask in blender forum?
First off, thanks for asking why.
Your question was put on hold because it asks for an entire tutorial. The Q&A format of stackexchange works best when the asker asks a question specific enough to be answered directly, without requiring explanations of numerous sub-topics.
The point is, questions which start from scratch "how to make <thing>?" generally involve too many aspects to cover in one answer. A much more stackexchange-friendly question might be "how to make <thing> a different color?".
All that said, the above still doesn't help you make a fireball ;)
If you are not sure where to start, there are plenty of blender and unity tutorials out there. In this case, I'd suggest looking for unity tutorials, as that is what you'll want the fireball to be in at the end of the day. A quick search turns up quite a few promising results. There's also some popular blender resources linked in the Beginner's Reference, but a search engine is your friend here.
So, no worries, and good luck!
If you hit a wall on the blender side of things, then please do come back and ask a question about it then.