Alright you guys are steering him towards the 'drink but be safe', my advice is a little different.
If you have drank before, then you should know or be close to knowing your limit. You need to get DRUNK, not blackout drunk because that's not fun but seriously just wasted. It's your 21st, your friends can take care of you(Make sure you have a DD and if not, call a cab).
But my biggest advice is plan the bars you want to hit up. A lot, if not all, bars have drink specials for that glorious day and make sure you use it wisely and don't waste the opportunity. You will come to realize most bars cost a crap ton of money, it's expensive to drink out, so you need to become one with drink specials.
PS- Make sure to tell everyone it's your birthday, everyone will cheer, buy you more drinks or give you a hug/kiss.