i just brought a domain name but dont have the site up yet. is it good to start building backlinks now or wait till the site is up?
Wait until the site gets up. Google might mark it as a 404 webpage (which may also cause a smite at your domain), and it will not be indexed after sometime it is check again. Better start creating a page with information text with your keywords, and start promoting the domain by having your links placed on someother's website.
wait until your site gets up. or you can start linking after having an index page with contact us or coming soon msg.
Publish at least few pages of content and build a few quality backlinks. Leave it like that and when you have time, develop the site and continue with the link building process.