I am launching one new site . So how can I build links? Should I submit them to several link directories or they are just waste of time? Please let me know..

Thats Correct. Also I would recommend to get a blog for ur site. You can promote,write, and share about ur site. Can get numerous backlinks and trackbacks. This is another great way to build links. I also think submitting to Directories doesn't help much and you need to submit ur site to 4000+ directories.

good blog! It's true what you said here "Never try to place hundreds of link for a new site on the very first day. Gradually increase the number of links." ...alot of people who tries to accelerate their traffic building by over doing it sometimes. THey forget that Google tries to look at sites that grows organically ...in as humanly possible...

Especially when your site is new, it takes weeks to gradually build your links and etc.  Imagine the newer your site is, the less likely the google bots will visit...