Think about what size dog you want, do you want a crossbreed or a purebred dog? Do you want a dog which needs lots of exercise or one which requires very little?
Then you have to think about whether you adopt an older dog or a younger one. In my experience most dogs under the age of one have ended up in a rescue centre because the owner could not cope with their behaviour. Remember than once a dog hits six months it's an adolescent and does what most adolescents do – misbehaves. From a year onwards this behaviour should settle but be prepared for anything before then!
Older dogs can also have issues – they can have health or behavioural problems which will have to be addressed.