I want to meditate but I don't know where to start.
Firstly I hate sitting up straight, its just not comfortable.
And secondly I don't understand what I am supposed to think about whilst meditating? or do I try to think of nothing? Is it OK for me to let my mind wander or not?
There's a lot of different ways to meditate, but here's one I can suggest. Keep in mind that this is something that takes practice - you have to train you mind over time before it starts feeling "right".
Start off by taking slow deep breaths, just lightly focusing on breathing and relaxing. I sometimes try to emulate the feeling you get when getting a good massage and sink into it. It feels nice and relaxing.
Once you feel nicely relaxed and your heartbeat has slowed down some, keep your mind fairly still and just passively "notice" anything that arises. If you start thinking about something random, like cleaning the kitchen or something, just notice it and let it go, like "OK, I'll clean later" or "that's a strange thought" and go back to your silence. It's OK to notice these things, but you don't want them to steal your awareness from meditation, so try not to put to much thought or feeling into them. Let them rise and then fall.
And that's it. It may sound boring, but as you get good at it and you hone your mind, it will become more pleasurable, you'll enter new states of consciousness, and it will become a source of energy for your daily life as well. And you'll become more intuitive and begin to know where you need to take it from here.
Also, I know you say you don't like sitting up straight, but meditation is all about training yourself. First off, you're training your mind to act in a way it has never done before. It fights the new patterns your trying to place on it and it gets uncomfortable - you might be tempted to say "oh, it's not working" and stop. It takes practice before you mind goes "oh, OK, this is a good thing". It's the same with your body. I'm not saying that you have to sit up, lying down is OK too, but training your body to get through a meditation can be the same fight as with the mind. It feels really good when your able to do something you never imagined yourself doing.
Hope this helps! Good luck.
