and Welcome to the forum,
Just a plan for a crossbow is a scarce thing and in most cases isn't enough to build one. A good comparison would be a violin...is it possible to build a good violin without any wood working skills or knowing how a violin works? Probably not, but there are places that can help you enter the world of such craftsmanship and The Arbalsit Guild is the right place to start.
Please read through the topics to get an idea of what makes crossbows tick, look at the works presented on this site, and of course you are always welcome to ask questions as you progress in your learning...remember - Everything is in the details.
The Arbalsit Guild
PS: there are a few plans available on our site in the Medieval Crossbow Reproductions
section, Crossbow Books
topic will also help with the understanding of the mechanics. And of course check out Lightly
's topics, she is a professional crossbow builder and has kept a picture record of her builds and thus has put together fantastic crossbow building tutorials.