Research has shown that giving birth in an upright position can be beneficial for both mother and baby for several reasons. The key thing to remember is to aim for a position where your torso is 'upright and forward' (i.e. standing, squatting, kneeling upright, or leaning forward on all fours) and to avoid lying on your back during labour for any length of time.
When the uterus contracts it tilts forward slightly so any position that is 'upright and forward' is working with your body (rather than fighting it and causing tension), allowing each contraction to be as effective as it can possibly be. Being upright also makes full use of gravity to help baby make their way down.
Lying on your back means that you are not allowing your pelvis the space that it needs to fully open, leaving less space for your baby to move down the birth canal, and therefore making things a little more difficult for yourself. The main blood vessel supplying oxygen to the uterus runs down your back and applying too much pressure on this risks compromising the oxygen supply to your baby.
You may be asked to lie on your back for any examinations. This is absolutely fine, and nothing to worry about. Just make sure that you move from that position afterwards - it can be easy to forget and stay put.
Listen to your body and move into positions that feel right for you. Don't feel like you have to be 'active' (swaying/dancing/walking) if you are tired, lying on your side is fine if it feels comfortable to you. Just remember that staying in same position for too long can lead to achey muscles and cramps so make sure that you change your position regularly.
You can print out the birth positions hand out below and practise some of these positions in the weeks running up to your due date to see what feels comfortable to you. You can take this handout with you in your hospital bag or remember that you can also ask your midwife for help finding a position that is comfortable for you during labour.