Sakis3G website was down for some time but is now back.
Visit the download page here and run it on your RPi using
You may need to try several times because the script will ask on which "port" the modem is.
Once you'll have the right parameters, you should be able to script the sakis3g using ./sakis3g start. If it does not work at first time, it will help you to provide the right arguments according to the right parameters.
Also, you will most probably need a powered USB HUB because 3G modems need a lot of electrical power to work, and RPi does not have enough (you will see it at once, the RPi will reboot in this case).
Be sure your carrier network is available, I had some issues because of this.
If this does not work, you also can follow the instructions given here but also can find some useful information here (even if it is a page for ArchLinux, not Debian).
If you need to configure it manually, below is the wvdial.conf I configured for a friend on its RPi:
Modem = /dev/ttyUSB3
Dial Attempts = 1
Dial Command = ATM0L0DT
Baud = 460800
Modem Type = USB Modem
Check Def Route = on
Abort on busy = off
Abort On No Dialtone = on
Ask Password = off
Init = ATZ
Username = websfr
Password = websfr
Phone = *99***1#
;Phone = *99#
Init1 = ATZ
Init2 = AT+cgdcont=,"IP","websfr"
Init3 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
Auto Reconnect = on
ISDN = off
Auto DNS = on
Carrier on check = off
Idle Seconds = 1000
Stupid mode = off
Replace 'websfr' and phone number with information from your carrier.
Replace /dev/ttyUSB3 with /dev/ttyUSBx (x can vary according to your modem, you'll have to try it).
And here is a package that contains information to help using usb-modeswitch.
You may also need to reset your usb device sometimes. Instead of rebooting, you can use the tool given there to do this (you'll need to compile but instructions are straightforward).