I'm going to assume you're on a really tight budget, and that you're a complete beginner.
Get on eBay, Craigslist, your local small ads paper, etc. and look for a second hand electronic keyboard.
Don't worry about high quality piano sounds. Don't worry about weighted keys.
Do make sure the keys aren't stupidly small. Do make sure there are at least 60 keys. Do make sure it's "velocity sensitive" (that is, hitting a key harder makes a louder sound).
You should be able to find something for under US$50, and probably much less. There are 20-year-old keyboards that will be good enough for your immediate needs, as long as they're not broken.
A "keyboard controller" may be a good compromise for you, but be aware what you're getting into. These do not make a sound themselves - they send signals to a computer or a midi device. However, that means they can be cheaper, if you already have a computer that you're happy to use for the purpose. One downside of this is that there can be preparation time (plugging the keyboard in, booting the computer, loading the music software), that gets in the way of spur-of-the-moment practice.
This instrument would not be a good substitute for a piano in the long run, but it will let you get started on learning the basics. You should borrow time on other people's real pianos (school/church/teacher/friends/etc.), so you know the differences between your "toy" practice instrument and a real piano.
While you're practising and learning with your electronic keyboard, save up some money towards a "serious" instrument.
Alternatively, you may find you can make the music you want using an electronic instrument, and there's nothing wrong with that.