Daily Challenges change from day to day. To find your challenges, press , and scroll down to see Challenges. Select "Challenges" by pressing , and it will give you a detailed screen of what the challenges are for the day (as well as the weekly challenge at the bottom). It will show progress stats, and if you select that challenge, it will put you in the appropriate lobby to start working on the challenge. They are the four challenges at the top of the list. Complete all 4 challenges and you have completed all the daily challenges.
If you are unsure on what/which challenge is what, or how to complete the challenge, please refer to this: Daily/Weekly Challenges Thread