That's going to depend on what you already know!
If you know Java, then look at starting with Android Studio:
Download Android Studio and SDK Tools | Android Studio[
^]
If you know C#, then it may be worth looking at Xamarin, now it's free and part of Visual Studio 2015.
But...if you are running a 32 bit PC I suspect that you aren't going to enjoy the experience: android emulators are sluggish at the best of time, and a slow PC isn't going to help at all. And 32bit PC's are limited in memory, so that's really going to nail your performance to the floor. I'd strongly suggest that you at least consider upgrading your PC to 64 bit with a good amount of RAM and a 64 bit OS before you move any further.