You have to be a little more specific...
Do you mean how to you get on the internet? If so, you can click on the Safari icon...assuming you didn't rearrange your fixed icons it should be one of the 4 icons that always shows on your home screen at the bottom.
Do you mean how do you turn it back on after going into Airplane mode? Just go back to settings and turn Airplane mode off.
Are you having trouble getting the internet to work? Are you using WiFi? If so, try going to settings and turning WiFi off to see if that works. If it does there might be an issue with your WiFi setup. It might work again if you turn WiFi back on...if not you might need to check your WiFi setup. If it's an issue with 3G working...that is more difficult to diagnose. Could be an issue in your area. Try the usual (reset/restart/restore/dial *228 option 2) to see if that helps.
Just blindly throwing things out there since I really don't understand what you're asking.
I would like to assist you with restoring an internet data connection on your iPhone 4.
There are two ways you can connect to the web, one being via a WiFi connection and the other via the Verizon Wireless 3G network. I'm including instructions for turning on and setting up a WiFi connection in the links provided. If you're using the Verizon Wireless 3G network for internet data connections, just ensure data and data roaming are both enabled. This will ensure your device is ready to connect to the data network.
Please let me know how this works out for you.