Is there a particular posture I should use?
Similar to finding a meditation place, you can experiment with different postures. One posture is not better than another. Sitting in a chair is fine, as is sitting in a cross-legged posture. The important thing is to respect your body and do your sitting, standing, walking, or lying down meditation in a way that balances relaxation and alertness.
That said, here are some helpful tips for sitting posture:
Sit in a way that allows the spine to be upright and relaxed, following its natural curves.
Allow the shoulders to relax back and down.
Place the hands on the thighs or in the lap (perhaps resting on a small cushion or towel) and allow the arms to relax.
Allow the back of the neck to lengthen and the chin to slightly tuck in.