You didn't really define "decent" Do you want someone to be able to play sheet music? Write your own songs? Play by ear?
How to learn to play:
1. Get more time.
2. Remember which notes on the paper go with which keys or play by ear.
3. Practice.
4. Um...
5. Ect.
6. Profit?
With sheet music:
Find a good sheet music store, because the more songs you have access to that you like, the better. If it says "Easy" on it, it means "Crapified" Lots of modern day musicians release crapified sheet music. Sometimes it takes forever to find it uncrapified. If there is a song that you really want to play that is hard just practice little bits over and over until it is good. I think it's less frustrating to just pick a simplier song.
http://www.music-scores.com/piano/composer.php That site is pretty cool if you want some free sheet music. A lot of them are just for members but there's enough free ones to occupy you for a while. Never pay full price for classical sheet music. The guy has been dead for a long time. You can find it if you are crafty enough.
Song writing?
If you just wanted to write your own songs, buy one of those basic piano books with the chord wheels and practice a bunch of differnt chords and scales and stuff. Same if you don't remember basic piano stuff like which keys go to which notes.
By ear:
Listen to stuff over and over. Play it on the keys.
Summary:
Pick things that you enjoy the sound of, practice lots, don't get frustrated. The easist way to start is probably classical, because if you are playing by ear, there usually aren't a lot of extra instruments in the way, and you can find a lot of free sheet music.
I miss my keybord!

5% of that should be helpful, you decide which!