Your Strength score increases by 2, and your Charisma score increases by 1.

How old is magic? That's how old you are!

Who even bothered asking you? Alicorns are a paramount of goodness! (most of the time)

You're big enough for someone to ride, so your size is Large. This means you use a 10ft. by 10ft. token when a battlemap is in play. 
See page 191 of the Player's Handbook for more details.

Your base walking speed is 40 feet, and your Flying speed is 40 feet as well!

Your creature type is fey, rather than humanoid.

If you move at least 30 feet straight toward a target and then hit it with a melee weapon attack on the same turn, you can immediately follow that attack with a bonus action, making one attack against the target with your hooves.

Your hooves are natural melee weapons, which you can use to make unarmed strikes. If you hit with them, you deal bludgeoning damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.

Any climb that requires hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

You can speak, read, and write Common and Celestial. Alicorns love sharing gossip in Celestial. 

Accustomed to twilit forests and the night sky, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

As you are a most wondrous Alicorn, you only have four hooves and not thumbs. You can still perform the Somatic component of spells using mane weaving and tail swishes, but you cannot open doors or hold a weapon unless it is strapped to your head. You can't open doors, chests, or I mean really anything that requires thumbs. Best of Luck to you!