I am having Casio CTK 810 indian keyboard. It has an output option of USB and audio out. My question is, how can I connect the keyboard to my computer and record and pls tell me which recording software is right one for my keyboard?
If it has USB out then you can use it with any software of your choice. Just go to preferences of the program and you can set it up there. The audio out is for connecting to speakers for live performances. Try the program GarageBand, it is very cheap and suitable for a person who wants to record their piano playing. The only problem that you may encounter is that it is only for Mac computers. If you are on a PC try the program Fruity Loops, it is very easy to learn and comes for a low price.
It's not clear what the USB connection does - it could send/receive MIDI data, or it could send audio. If it doesn't send audio, you'll need an audio interface which you can use to record using the outputs on your Casio. Your computer may have line-level audio input (not a microphone input!) that you can use although the "quality" of the recording is generally not as high.
Any recording software you like should work just fine. Audacity is reasonably simple, free, and popular, but there are many others.