How do I configure which programs autostart when connecting to the internet? Right now skype starts, email clients send/receive.

I really don't want skype to start and login when I connect to 3G, but it's OK if I connect to wifi. 

How do I configure which programs autostart when connecting to 3G / wifi?

(Sony Ericsson Xperia X8, NOT rooted)
Without root you only have the control the app developers give you: If an app allows you to configure this behaviour, you can do so -- otherwise you can't.

If your device were rooted, on the other hand, you'ld have several possibilities:


Use DroidWall to define which apps may use 3g, which Wifi, which both, and which none (1st pic below)
Use LBE Privacy Guard to do the same, and more (2nd pic)
Use Autorun Manager to disable certain listeners of apps, so they no longer get informed about network changes (3rd pic; experienced users only!)


  

This is just a selection, and thus the list is very incomplete. As you already wrote your device is not rooted, it doesn't make sense to put all those details here; it's just to give you an idea what would be possible with root, not to elaborate all options ;)