The braces themselves are the little metal pieces that they plaster on the outsides of your teeth to which the adjustment wire is attached. Those things are fine and generally comfortable so long as the braces themselves aren't jagged. The first month it will feel like your mouth is constantly full as the metal presses against the back of your lips, but you adjust to it within a month. Just don't get stuff like caramel or taffy stuck in it, because it'll drive you nuts.
The adjustment wire is what you really should be worrying about. Basically it's the force behind the braces, the very thing that gradually shifts your teeth around with constant brute force. When you get your braces in for the first time, your mouth will feel sore to the point that anything even touching your teeth will cause pain for a week. Invest in soft foods (applesauce, vienna sausages, ice cream smoothies) and start training yourself to eat without chewing ( I found that holding food between your molars and tearing with your tongue to be quite effective), and you'll survive. The pain will subside as you grow used to the wire, and you'll be back to chewing in no time. However, note that you'll be getting a new wire periodically for further treatment, so this cycle will repeat every few months until your treatment is complete. Thankfully you will get used to it and it'll get easier every time; by the time I was done with my 1.5 year-long treatment, tooth soreness was gone within 2 days, and I was easily able to remain comfortable during that time.