http://developer.android.com/index.html

If you have experience writing Java, try writing your own launcher or something. I wouldn't suggest building an OS from scratch (even if it is based on Android), unless you have a lot of time, dedication or a team.

If you really mean an alternate OS; look at http://bitbucket.org/floren/inferno/
If you mean something that isn't Android, then I'm sure you'll be just as happier getting rid of the Java UI and installing a DE on that shit.
People have already installed Debian on Android phones too; http://youtu.be/X9zwd3r6UQQ

I've never developed for Android, but can't you install the Android emulator and test the OS on it? After it's working you only have to copy it to the phone (and then make it work again...).
Or you want help on how to write an OS? Operating Systems Design and Implementation by Tanenbaum would be a good start (unless you dislike Tanenbaum's writting style).

Writting an OS is hard work, if you aren't willing to spend a lot of time on it give up. It's probably harder than you're thinking. That said, it isn't impossible either.
Oh, and remember: "Those who don't understand Linux are doomed to reinvent it, poorly."

you can always download the android source code and see what makes it tick and even modify it to suit any specific needs you have. writing a new OS would be time consuming and pointless (unless you're trying to impress some people) and will probably need small team of programmers and engineers just to make something decent in today's standards. so open source is your friend and best bet!

>>255

Rooting your phone is the first place you want to start. Then try installing some custom Android OSes, CyanogenMod is a modification of Gingerbread (Android 3) and is the most popular and very customisable but is also pretty resource intensive so it might not work too well for you.

I have a HTC Desire that I run Oxygen on. Oxygen is another mod of Gingerbread, very similar to CM but runs a lot smoother IMO.

Also, you can download Android OSes from Google for free and run them in virtual machines like Oracle's Virtual Box. I'm pretty sure you can get the source code too.

On another note, apparently Anroid is supposed to be merging back with the main Linux kernel pretty soon so that might be worth looking into.

>>294
Gingerbread is Android 2.3. I hear CM already has ICS (4.0) builds though.