Use a VPN to add a layer of security for your data. Set up strict setting as VPN can also monitor your activity.
Download all your data (or request an archive) from all the services you've ever signed up for before deleting your account or data - this way you'll have a better idea of what they have on you. A lot of the data is also useful to you, maybe not now but in the future.
Search for tools and services that let you opt out of internet trackers (e.g. Digital Advertising Alliance's Your Ad Choices tool)