If you're ready for something, you're prepared — you're all set to do it. You're not truly ready for bed until you're wearing pajamas and have finished brushing your teeth.
You can be physically ready, like when you grab a towel and sunblock to get ready for a day at the beach, or mentally ready, like when you're ready to rock your algebra test since you studied so much. Other things are ready when they're available, like dinner or a hotel room. If you ready something (or yourself), you prepare for some specific purpose. The Old English ræde meant "prepare a horse for riding," from a root meaning "ride."