Hi,
I asked about which yoga mat to get last week, so I thought I'd share my thoughts. I recommend these mats by Yoga Matters. We used them in my teacher training. They offer a good level of cushioning to protect your joints, but not so much that they become too thick, which could de stabilise you in balancing poses.
They're also sticky, which helps to prevent slipping.
They're £26 from yogamatters.com or Amazon.
Enjoy your practice!