Drive sprockets

I've purchased an OEM 40T rear drive sprocket.

Now I need to get a front sprocket, but I wanted to ask you guys some questions before I do.. just so I can learn a bit.

Are there certain size sprocket combinations that are required? Like 16/40?

Or can I use a different front sprocket with a 40 tooth back sprocket.

What will give me better mpg, or lower RPMs at higher speeds?

I'm getting confused with this stuff

Thanks!