Hi Paolo,
You'll better give us more information about your antenna. To begin with, to match the impedance to 50 ohm you will need a matching network. If your antenna is a narrow-band antenna, this network can be made with a RC circuit, commonly built using parallel stubs. If otherwise your antenna is a wide-band antenna, covering from 2 to 4 GHz, then a wideband matching network is needed (a bit more complex and less efficient). Tapered lines are the most common options.
I hope it will help you, and if you need more help, simply ask!