Well I did learn the guitar by myself, mostly through books, listening, persistence, and getting tabs online. I learnt the keyboard a bit, managed to play silent night and ode to joy, but then I got bored of it. Thought I'd just stick with the old guitar.
Finding an instructor wouldn't be such a bad thing, because you'll learn faster and won't have too many bad habits, but it's not necessary. I know a few who just thought themselves.
Here's how I'd do it though if I were you:
1. Get a For Dummies Book.
You'll get some basic info on the keyboard, how to sit, how to hold hands, some theory, etc.
I can't tell you how helpful it was for me when I just began learning the guitar. Even thought me how to change strings. Yay! Or find a friend to teach you the basics. People are surprisingly helpful.
2. Have a basic idea of what songs you'd like to play, and how good you want to get.
So you'll know where the hell you're going and why you're practicing. Classical or more contemporary music? Very demanding pieces or just some simple tunes?
3. Spend some time on it.
I'd recommend at least 30 minutes a day. If you're lazy, split it into 15-15 or 10-10-10. A little bit everyday is way better than a lot once a week.
4. Be patient with yourself, and start as slow as you need
Your fingers haven't really memorized the movements, so when you just start, just move your hands really slow. Don't try to rush. A good thing to remember is that precision can't be added to speed, but speed can be added to precision. So just increase the speed incrementally, and enjoy your snail paced version of songs.
It's only temporary, so don't be disheartened.
5. Fool around with your keyboard.
Try to figure out some simple melodies on your keyboard. Just use the white keys for this.
I think that's pretty much it. And remember to enjoy it. That's why you're learning to play, right?