how can I auto connect to open wifi? I have Galaxy Note2.

Is there a way to tell it to auto connect to open wifi when availble?

Whitout me needing to scan and choose open wifi?
In your devices settings (under Settings &rarr; WiFi or Settings &rarr; Wireless &amp; Networks &rarr; WiFi, depending on your Android version / ROM) you should find a check-box to enable notifications for "open networks". If you enable that (and, of course, have WiFi enabled), Android will always scan for networks in the background (which it does anyway with WiFi enabled) and display a notification in the notification-bar to inform you whenever an "open network" was detected.

This does not automatically connect you to that network, for security reasons. Otherwise hackers would have an easy job to lurk you into their honey-pot, hack your device, and leave you - virtually - "naked". It's not a really good idea to simply connect to any open network around. Many background connections are not secured, or at least not well secured, so it would be easy to sniff your passwords or personal data transmitted (same for your "manual connections" via e.g. the browser). So pick your "open networks" carefully.

However: once you manually connected a network, Android will remember that -- and connect you automatically the next time it is in reach. Until you go and remove its configuration manually from the settings.

There are also apps available on the Playstore helping you in this context. While Android's default scanner shows networks using a "captive login" as open as others not using it, they even differentiate. A good example for those is e.g. WeFi Pro (at least it did show these differences in the past -- it's some time ago when I used it last).