Get redsnow from the Dev-Team blog and download the ipsw 4.3.3 from Internet. Perform a backup of your phone using iTunes, make sure you do not accept to upgrade to the latest iOS if iTunes ask you to do so. When you done backing up your phone files, close iTunes and run redsnow, follow redsnow onscreen instructions to jailbreak your phone. The worry over here is that if you don't read everything very well and happen to put the phone in recovery mode, iTunes will force you to upgrade to the latest iOS. You are going to be stock with it, so do read very well the instruction in redsnow 2 3 times before performing actions either on the device or on the computer. Also make sure you runing redsnow with admin privileges of your computer. You can do that by login into your computer as the administrator of the computer.