when i change my SIM card, my iPhone does not work, it says you have to activate it

Activate what?

What exactly is the issue?

1 - if you mean you cannot do phone calls, you want to activate your sim card with your sim card provider. nothing to do with apple.

2 - if your iphone does not work at all, you want to connect it to itunes on your computer and follow the instructions. BUT FIRST, put in your old sim card, connect to itunes and do a backup on your computer. then change sim cards again and reconnect to itunes to activate.

Please post exactly what the message says

It is hard to guess based on your posts.

You've encountered activation lock. You MUST use the credentials for the iCloud account that was previously used to sign into iCloud and turn on Find My iPhone.

If you bought it used, contact whoever you got it from. They will have to remove the lock before it can be activated and used.

There is no way to bypass the activation lock.

Sorry? What is turned on?

I think you need to read this: Find My iPhone Activation Lock - Apple Support

It will explain what it's asking for and why.

how to activate