I am new to this community, this might feel dumb to you 
and i have one question if i have hosted a file do i have to be online all time so that others can download that file if nobody else has hosted it.
thanks 
In short, yes. However, you can, e.g., pay a pinning service (google “ipfs pinning service”). In the future, you’ll be able to use Filecoin to do this.
Over time, I expect that free services will pop up for hosting content as a public service (free artwork/learning material, open source code, etc.). However, I don’t know of any such services at the moment.
Thanks for taking the time and answering my question.