Will I lose my pictures and videos stored on my iPhone if I change my password code since I don’t remember my old one

If one had attempted unlocking multiple times with incorrect Lock Screen passcode, or if a message says that the device is disabled, learn what one needs, what one gets, and what is lost.

You need a computer to follow these steps. If you don't have a computer, borrow one from a friend.

Forgot? Click below to reset…

Click->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Regarding your data, If…

1. You have an iCloud or iTunes latest backup, then there is nothing to worry about. If not read more...
2. You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive; like the information that are synced and stored in iCloud are, Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive. (*-If you had turned on iCloud Photos, your content is automatically stored in iCloud.)
3. You have not backed up or do not have the latest backup, nor imported nor you have synced nor uploaded your data to cloud drives prior to the situation that you are in, then I am sorry, all your data is lost.

No, you won't lose anything unless the iPhone is disabled.

Click->If you forgot your Apple ID password - Apple Support

Click->If you forgot your Apple ID - Apple Support

If you delete Photos / Notes / Voice Memos / Videos / Reminders from the iPhone it is deleted from iCloud too, and vice-versa. Alternatively, try uploading to iCloud Drive then you can safely delete it from the iPhone. 

It all depends on how you need to change your passcode. If you can’t access your phone because you can’t remember your current passcode, you will need to restore your phone through your computer which will erase everything on your phone. So, yes you might lose everything without a backup. Can you access your phone at this time?