Be brave and participate. The more you put into it, the more you will get out of it.
It is natural to feel nervous about speaking in front of your seminar group or sharing your views. Remember that seminars are not necessarily about getting the right answers and having the perfect ideas. They are about experimenting, taking risks and raising questions. If you don't venture your thoughts, you will miss out on that whole process.
Remember that other people may be nervous too, and you are all at the same level of your studies, wanting to learn as much as possible. Your tutor also wants you to learn, and will appreciate your input into the seminar.
Try being the first to speak! This will get your first contribution out of the way, and it will make you feel good to be the one to get the discussion started.
Listen to others. Nerves can sometimes make you focus only on the thing you want to say. Make sure you are also listening to what others are saying. Be flexible and move with the discussion. You can learn a lot from other people, and from the act of discussing and debating.
Seminars are a chance to test out your arguments on a live audience who will respond to and develop your points. This will also help with assignment writing.