How do I tie up tomatoes, and which pest did this (pic)?

That would be a tomato worm 🐛

Use natural twine not tight make a loose loop and hook on your support so if it grows more the twine can slide up the support

The mess is the droppings of a tomato hornworm. It's exactly the same color green as the tomato vine. They get pretty big by this time of the year - you just pull them off (use gloves if you're squeamish) and I toss them in the grass for the birds to find. For the vine, I usually use cotton twine - like kitchen twine - and make a couple of loose loops around the vine (above and below a side stem) and tie that to the cage or stake. Remember tomatoes won't set fruit if the temp reaches 90' in the day.

Check this out and see if it helps: https://www.hometalk.com/diy/outdoor/pest-repeller/controlling-potato-bugs-and-tomato-worms-organically-in-the-garden-4071839.

Controlling Potato Bugs and Tomato Worms Organically in the Garden

I agree with others, tomato horn worm. Best time to pull them off is at nite with a flashlight. At least that is what my momma always said and made us do. We used them for fishing though they are kinda gross (I was a kid). If you have a pond I bet the fish would love them if you threw them in.

Tomato Hornworm. If there are white eggs on the tomato worm, they are Braconid Wasp eggs. They are eating the worm. https://aggie-horticulture.tamu.edu/galveston/beneficials/beneficial-04_braconid_wasp_on_hornworm.htm

Here's a post that might help - https://chascrazycreations.com/keep-pests-out-of-your-garden/