I am lucky in that where I live (Al Ain, UAE) there is a very active natural history group, which organises field trips and lectures on all sorts of interesting subjects. I have blogged on these trips before and I record what I see on field trips in my online notebook.
But even if you live in the city there may be a conservation group that you can join to both meet like minded people and explore the greener parts of your city.
Most counties in the UK have a dedicated Wildlife Trust and you can find your local trust here
The wildlife trusts helps people to engage with British Wildlife and learn about conservation through activities such as surveying wildlife. They also manage nature reserves and publish books and reports on their activities.
If bats are your thing there are a lot of regional Bat Groups under the UK Bat Conservation Trust umbrella. You can do training courses that will allow you to handle bats (in the UK you need a licence to do so) and they also run the bat helpline that non Batworkers can call if they are having problems with bats or have found an injured one.
The Mammal Society is all about the study of British mammals and not only can you participate in field work such as surveying, but they also run training courses on various aspects of mammal study such as radio tracking or identification.
The Amateur Entomological Society has fairs and local meetings if you are interested in insects as well as publishing a scientific journal. As with most of these societies you can take part in record keeping and surveying, as well general conservation efforts and raising awareness of British insects.
Finally there is the British Herpetological Society dedicated to the conservation of our native reptiles and amphibians. This group also publish a quarterly academic journal, so great for those really dedicated to Herpetology.
If your tastes are more exotic then there are hobbyist's societies such as the International Herpetology Society and the British Tarantula Society that can cater for you.
These groups have lectures, and often arrange trips to Zoos and shows, where you can meet even more people and get involved.
Tell me about what you do to get involved.