We have the power to use our voices to influence companies and governments. While we may seem small individually, the additive effect of many people does make a huge difference. Think about supermarkets moving away from plastic bags; this would never have happened without mass public pressure.
Social media can be a very useful tool to getting your voice heard. You could post about issues tagging companies involved. Perhaps you could do a vlog about a subject that interests you. Following environmental accounts and charities is a good way of keeping up to date with projects that you could get involved in.
Signing petitions can make governments listen to the public. You can find petitions that you are interested in by following charities or by signing up to petition sites such as Change.org or on Gov.uk. You could also start your own petition on these websites.
Writing to your MP is a valuable way to have your voice heard. Sometimes charities will write template letters that you can adapt about a specific problem, or you could write your own.
Companies do listen to their consumers so if there is a practice or product that you feel is harmful towards people or the environment, let them know. You can send an email or phone them or even call them out on social media. Ask your friends and family to do the same.