When you first enter your garage/home, press the start button. It will take you to the screen where you tune your car. Also, before a race, you can select the red toolbox to go to the settings screen. When you get there, select the tool icon next to the area which shows you what suspension you have equipped.
And welcome to GTPlanet. Be sure to use the search function before you post a question. It's the icon that has a small magnifying glass nest to "search". Usually, I find what I'm looking for much quicker then posting a question and waiting for people to answer.
Hope this helps,
Duck7892