One piece of free advice: First, do no harm. That means don't go arbitrarily changing stuff until you have done your homework.
Yes, the previous admin should have left some buffer for Static IP's and gateways. Don't be like him. Train yourself in the fine art of network administration. Don't expect anyone else to train you, get some books, compile references and don't be afraid to ask questions.
Here's where you might start - I know it sucks, but there's not really any other substitute for knowing your stuff in IOS.You just have to knuckle down and get a fundemental understanding of the commands and setups.
http://www.cisco.com/en/US/docs/ios/preface/aboutios.html
Good luck!
