there is a lot of romance regarding this period in history but it was a rather miserable time to have lived unless you were born of high ranking noble blood.

Without the support of modern infrastructure to which you in the present have been conditioned to live (especially sanitation) there would be immediate issues.

Any water you drink would most likely result in you getting dysentery ( there is a reason they tell you to stay away from loose water in third world countries) Any infection would be dangerous, as you do not have the luxury of antibiotics to treat them.

Your so called knowledge would not be of a lot of use, you could go on for hours about your smart phone, but with out the supporting technology that had to have developed over the centuries you could not ever hope to build one. Unfortunately an average person living in the 21st century is conditioned to get their food at a store, most of us could not really grow our own, nor hunt game. If you live in town, you would have to acquire a skilled trade in order to survive economically.

If you do not adhere to Christianity, you would be branded as a heretic and summarily executed. In fact you would not really have any legal rights in the sense you have them today (the magna carta only applied to Nobles not commoners)

As for fighting in battles, your odds of surviving would be slim, the majority of soldiers usually got killed during war, even knights did not have a long lifespan.

I have nothing against renaissance fairs, it is fun to see people dress up and reenact the middle ages, but then when it is over, you get in your car( climate controlled) drive quickly home ( a journey that would have taken days by horse grab a drink from your fridge, and play some Assassins Creed Brotherhood and experience Italy circa 1490s in all its digitally recreated splender.