'My friend told me that it is better to position my bed / table against the wall so that there will be someone to back me up in whatever I do. How true is that ??

Hi Jackson, it is practical to position the bed or table against a wall. However, this is not a guarantee of meeting 'helpful people'. Feng Shui is about Qi, so what you can do is to sleep/sit facing your 'Tian Yi' direction, based on your Gua number. Then you can receive 'helpful people' Qi. Cheers! SN