Hopefully if you went to a professional they wouldnt nick your idea.
Have you had a look at setting a website up yourself? Depending on the scale of your idea that might be enough for you, or at least would give you a better idea of what you need to ask of the person who is setting it up for you.
You can't copyright an idea, and you can't patent software or services.
So the way to protect your interest is by signing a contract with confidentiality provisions which will protect your interest - any professional (company or individual) will have these terms in their standard services agreement, but do check the agreement carefully or have a lawyer do it for you.