I have an iPhone, but most of what I say is applicable to other phones as well.
You may find the function I reference in a different part of your phone though.
7 Ways To Clean Up Your Phone
1. The first thing is the same advice for changing anything on your computer. Back it up before you do anything else. So backup your phone either to your computer or to the cloud. Just follow the prompts that will guide you through the process.
2. Delete apps you don't use. If you haven't used an app in several months, chances are you really don't need it so delete it.
3. Pictures are what usually eats up the phone's memory. Offload your photos to iCloud or another website.
There are several that help you accomplish this like Amazon Photo, which, if you're a Prime member, is free I believe.
Google Photo and Dropbox are other storage sites. Usually, you're offered a free amount of storage. If you want more than that, fees apply.
You can also store them on your computer or a backup drive.
4. Saved texts eat up storage too especially if there are photos sent with the text. Save the photo first then go to Settings. You should have an auto-delete function for texts older than a year or another specified time period. Delete them.
5. If you often use your phone to go online, go to your web browser settings and clear the cookies and history.
6. In Settings, you can turn off location services for apps you don't use often. Just turn that app's location service back on if you want to use it.
7. Also in Settings, you can disable auto-updates. That way it's not constantly sucking battery power on a daily basis and eating up memory. Update the apps when you need to do it.
Sometimes a few common sense tips can save you money.