cause i can't get facebook and other things someone help me?
Although the official Facebook app doesn't work with the iPhone 3G, there are some apps which claim they are compatible. Here are two you can try:
There are a lot of apps and games which are compatible with your older device. You just need a tool to separate them from the incompatible apps.
Here is my favorite solution:
iOSSearch - search the iTunes store for compatible apps.
And here are some things other people have suggested:
Vintapps 3.1.3 - paid app.
Apple Club - filter apps by iOS version.