Usually the first thing you need to do is to try and establish who owns the land in question. If it is privately owned and has no history of public access or public use it may be that your only avenue for influencing is through the planning process. Find out if there is a local plan or neighbourhood plan covering the area, see if the proposed development is compliant with those plans and engage with the public consultation process when planning permission is sought. You might also be able to mount a public campaign if your concerns are widely shared amongst the community.
If the land is publicly owned or has a history of public access or community use you may be able to register the site as an asset of community value. This would give you some rights to try to purchase the land if it is sold in the future. Alternatively you could try to secure ownership of the land through a community asset transfer or protect it by getting it registered as a village green.
The My Community website has lots of information about asset transfers and registering an asset of community value. The Open Spaces Society can help you with protecting village greens, commons and public paths, and Fields In Trust can provide support and resources for protecting parks and playing fields.
Unfortunately we are not funded to provide free advice and support to groups. We usually provide paid consultancy support either directly or as part of a wider funding programme but please do get in touch if you think we might be able to help you.