There are lots of ways you can help be a responsible tourist. Here are just a few ideas:
Visit less popular destinations. Overtourism is increasingly becoming an issue, so visiting places which welcome fewer visitors will help alleviate those destinations which are overcrowded but also benefit locals which depend on people like you visiting their country. You'll have a much better experience as well!
Visit out of season. Of course, if you want to visit places which are people, then that isn't a problem. However, we'd recommend visiting out of peak season. The weather is often never as bad as you think and the crowds will be much thinner.
Stay in eco-friendly hotels. This is becoming easier, with more and more hotels focusing on wide-ranging sustainable initiatives, from the likes of generating their own power to growing their own food and wildlife conservation.