i downloaded itunes on my computer to transfure my stuff from my 6 s to my new iphone 8 how do i do this

tt2's instructions predicate you actually need to get things from the 6s. Ideally this should not be necessary and it should all be on your computer as a coomon source from which you sync both devices.

Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support - https://support.apple.com/HT201253 - detailed example: https://discussions.apple.com/thread/4090536?answerId=18860187022#18860187022

Sync your iPhone, iPad and iPod touch with iTunes using Wi-Fi - Apple Support - https://support.apple.com/HT203075

iTunes 12 for Mac: Sync iPod, iPhone, or iPad - https://support.apple.com/kb/PH19458 - including manual syncing

If you've used the 6s independently of a computer prior to getting the 8 then the general approach given in Niel's link - Transfer content from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support - of backing up the old device to either iTunes or iCloud, and then restoring that backup to the new device will be fine.

If you've previously uploaded content to the 6s using a library which isn't on your current computer, and which you don't want to lose, then recover that data to an iTunes library, backup old the device, then restore that backup to your new device.

how do I store it to

I'm afraid I don't know what you mean by this question.

tt2

Click here and follow the instructions.

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See Recover your iTunes library from your iPod or iOS device. Once everything is stored in your local library you can add it to any device.

tt2