For most people without a lot of technical skills and information, keeping data secure and private can be a daunting task.
Here are a few basic things that can help keep your data and content more private. Remember that nothing is fool-proof and there is always a risk but you can do things to minimise that risk.
If your phone or computer is stolen and hacked, security and encryption are only partially effective protection. Therefore, it's important to make sure your device is physically secure. If your phone, tablet, or computer supports it, turn on full-device encryption and use short time outs on lock screens. These can usually be found in the "settings" menu. Don't be intimidated to ask for help, doing a simple search can help you find easy to follow instructions.
Policy of Service
Spend an extra minute or two reading the policies of the apps you use. Do they have any mechanisms for complaints or redaction if things go wrong? A lot of popular free services have data policies that essentially mean the company owns all the rights to your images and content and can use it as they like even if you think it's deleted. If you are not comfortable with the app company owning your images or data, there is almost always an alternative service. You may just have to do a little more searching but it is probably worth it to educate yourself.
Ideally data should be encrypted as it passes between devices (in transit) and encrypted when it arrives (at rest). If you really want to be secure, look for so-called "zero knowledge" services in which the service provider can't technically read or view your message even if they wanted to.
Number of Copies
You may have deleted that file or picture, but is it completely deleted? Many phones automatically upload all images to cloud services. Have a look at your backup and cloud storage settings, you may not want those backups to happen automatically. Also, when a file is deleted, it's often just marked as unused, it's important to "empty the trash" to ensure it's deleted. If you send sensitive content or images to others, ask them to do the same.
Most of the time people exchange information or content with other people who know something about who they are. However, this doesn't mean they have to know everything. Think about the information you use in accounts and profiles and how those profiles are linked to other sites. This also applies to usernames, email addresses and phone numbers. Ideally, use services that don't require you to create an account or to create an account with minimal information that can't be easily linked to other accounts.