Google gives these steps
Try these steps to fix your website:
Register and verify your site in Google’s Webmaster Tools.
Sign in to Webmaster Tools and check the "Security Issues" section to see details of sample URLs that might be hacked. Fix the security issue that allowed your website to be infected. Otherwise, your site is likely to be reinfected.
Read our resources for hacked sites for detailed information on how to fix your website.
Request a review in the Security Issues section in Webmaster Tools when your entire website is clean and secure. After we check that your site is fixed, we'll remove the "This site may be hacked" message.