I want to make a torrent search engine which will provide links to other torrent sites. So I need data from other sites to index them in the database. So, is it legal to crawl a website for this purpose or is there some other way to do that.
Depending on the site, without permission it is not legal.
You might wish to investigate Common Crawl, a website that has already crawled the entirety of the web. Check out their ToU section to check on the legality of it all.