Should I take credit from the idea that I have initially suggested? I had an idea for a project and discussed it with my professor. 
The professor said that it was a new idea and we could try it. We agreed that we would first research more about the feasibility of the idea. The professor would be the project head/guide for it with me and my friends as part of the team.

Now, he has given this project to his senior students.
He says that we still don't have much needed technical knowledge, so the senior project members will be of help.
This is correct.

However, I feel we, sophomores, will just be sidekicks in the process and will lose out as major contributors to the project, although we will be part of the team that will be involved in modifications and development of the prototype .

The fact that we need help from seniors is very true and necessary

I have just discussed the basic mechanism but design of the individual components with the professor and the design of the whole model itself still has to be worked out.

Do I really lose out on giving away my idea? 

Thanks for all the answers and clarity.
EDIT: the original title of this post was "is this plagiarism?? what should i do now??"

To answer the original title question: this is not plagiarism. Plagiarism is presenting another author's ideas, text, etc as your own.
Since it seems from your description that you are still involved in the described project, no such thing has happened.

It seems more that your professor has taken the leadership of the project (which is not strange for a project running in his group), and taken decisions that make you feel less comfortable.

Your fear is that you will not receive (enough) credit for suggesting the original idea. I think that it is best to address this in an early stage (i.e., now) to your professor. Most likely he will assure you that you will receive credit for the original idea. Write down any verbal guarantees that he might give to you in an email to the professor. Then during the execution of the project, make sure that any senior project member is aware of any agreement between you and the professor (e.g., by forwarding the email).

Try to do this in a graceful and polite manner, and try to avoid appearing (passive) aggressive.