You definitely need to do some learning by searching the beginner's Jailbreak threads on here and Google. This question (and many others that will inevitably follow) have all been answered many times in depth by knowledgeable people on here.
But, to answer your question, if you JB your phone and ever need to take it into the Apple store for anything, the process is simple for avoiding any problems with the staff. First, you totally backup your phone (and your SHSH blobs, don't know what that is? That's why you need to research on your own). Next, once you're confident all your data is securely saved, you wipe your phone clean by Restoring As A New iPhone. After that, just take the device in and the employees can do whatever they want to, but they will never know that you were JB previously. After that, they'll either give you a new iPhone that you can restore to as your previous phone and re-JB, or you'll have the same phone that you can do the same to.
