I dont know much about Tik Tok. Can you help?

Hey.

I plan to use Tik Tok for sharing short videos I have taken on my phone.

What is the best way to format a post (also, can I upload previously taken videos?) and where do I put keywords (also the amount of it) to get the best results?

You bascially answered your own question in a way

You can learn that by going through the TikTok section here: https://www.blackhatworld.com/forums/tiktok.279/

Of course you can share your previous videos on Tik Tok. You click the plus sign to enter the shooting status, and then click the box in the lower left corner to upload your video.