The Bible actually gives us various examples of how people prayed, not only kneeling, but also standing or with hands raised or in other postures. When Jesus prayed in the Garden of Gethsemane just before His arrest and death, we are told He "fell with his face to the ground" (Matthew 26:39).
The important thing is not our physical posture when we pray, but the attitude of our hearts. In reality the two often go together; when we realize we actually are coming into God's presence when we pray, we won't be casually walking around looking at other things or doing something that distracts us (like watching TV or eating). True prayer requires concentration, and it also requires humility and faith.
After all, when we pray we're coming into the presence of the God of the universe, who is holy and glorious and all-powerful. But we also are coming into the presence of our loving heavenly father, who delights in hearing the prayers of His children. The Bible says, "Anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him" (Hebrews 11:6).
Never forget: Prayer is a great privilege -- a privilege that is possible because Jesus Christ gave His life for us. Even now, He is in heaven interceding for us. Do you know Him, and are you learning to walk with Him every day? God's promise is for you: "This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us" (1 John 5:14).