When we pray, we are talking to God. One to one on a personal level.
On our knees or sitting isn't important but the humbleness of our heart is. If you feel you need to be on your knees to try and find that humbleness...then on your knees is right for you.
I talk with God all day long and sometimes I slow down enough that I listen as well.
We should set aside a time that is used just for prayer (our talk with God). A time that we give Him our full attention. As our day goes on, we can keep in a constant communication as well. Some times our prayer may be short... Help! and other times our prayer may be extra long (on times we pour out our deepest thoughts and feelings with Him.)
As for praying out loud or just in your thoughts... you can do both and both would be acceptable.
The apostles asked Jesus... Teach us how to pray.
Jesus replied with an example that we know today as The Lord's Prayer.
It starts out with giving God glory... telling Him you know who He is.
Next we agree with Him... ask that His will be done and that means we are accepting of His will.
We then speak of our understanding that all we have and need comes from God... give me this day my daily needs.
Then we ask for His protection.
and any proper Hebrew prayer would end with giving God glory.
For thine is the Kingdom, the power, and the glory, for ever and ever.