Make sure you set your phone to remove location info on photos, messages etc... On an iPhone you go to Settings > Privacy > Location Services, and click yes or no for each. If it's an ex who has previously had access to your phone, make sure they haven't set your phone to 'share my location' with them. Your photos contain metadata which geolocate where it was taken etc.
Change passwords for everything, and also if you have a sort of pattern to your passwords, you'll need to change that too.
Make sure no one has access to your computer or phone. Even if locked, someone who was really determined (i.e. came prepared with certain spyware) can upload it in seconds and take control. That would be a serious stalker, or state actor though.
Lockdown all your social media accounts like FB, go through your friends and get rid of anyone you don't usually speak to, or don't know (the latter shouldn't be on there anyway!). Then set job, home town, age, birthday, etc to private only. Make sure your friends list is also private, so they can't get to you through friends. Change your name on FB so people can't casually search for you.
Limit what is on LI (on LI you can see who checks you out, unless they are a premium member or do an annoying work around). It's a balance between its use for you professionally and providing too much info to social engineering hackers or stalkers.
Twitter etc, just be careful what you post and make sure photos have no location data, don't check in to places. Other SM is beyond my knowledge. Block the person on every form of SM.
I have to be very secure for work, so we get trained by the very top people. But I did once realise a slightly weird and creepy colleague was showing too much interest when he said something he couldn't possibly have known without going on my FB account, and would have had to do quite an awkward work around to get access to it at all.
If you go on Future Learn there are often really good cyber security courses for free, some for 'normal' people rather than techies.