In the beginner tutorial section both camille and jonathan talks about standing upright and then practicing. Is it ok if I practice sitting upright ?

Because I don't know keyboard keys properly yet, so I have to look down if I practice standing and that won't be a upright position.

Hi, Ashutosh!

You can certainly practice sitting upright - just make sure that you have your feet on the floor, sitting on the edge of your chair/bench (so you're not leaning back), spine long, shoulders relaxed. Be especially careful about your chin reaching forward - that becomes an issue with seated posture more often than when you're standing. 

And if you're struggling to get enough support (for something like belting especially), stand up!