There are multiple antenna types that can be built for your FM tranmitter. Below are common ones that have with write ups from members:
Dipole - How to make a dipole FM transmitter antenna: Easy step-by-step construction of this valuable tool
1/4 Wave Ground Plane - 1/4 Wave Ground Plane by JHinkle: Discusses some antenna theory and how to build a 1/4 Wave Vertical Antenna
There is of course a lot of information else where on the web regarding FM transmitter antenna design and construction. Below are a few links that you may find useful:
VLP SWR Bridge
Tool designed by IdunBenhad to measure the SWR of the antenna and match it as closely as is possible to your transmitter.