Have you seen calving or lambing before? Get on youtube and watch loads, get a feel for what "normal" can look like.
In terms of signs to watch for, it varies by individual but in general watch for restlessness, annoyance / pain, separating self from herd for privacy. Later on look for water bag. Try and see which bit of the calf is poking out. If you see two feet the right way up, leave her to get on with it. If you see something wrong (feet upside down, back feet, bum) get help asap - it will take time for experienced help to arrive.
You'll need some way to restrain her if she does need help - but bear in mind you need to be able to release her if she goes down.
First timers often have a "WHAT THE **** IS THAT" moment. They seem to settle down once the calf suckles, but she might need restraining and the calf might need guiding in to the udder. Beware kicks from the cow!
I think it's worth agreeing a rundown / action plan with someone. E.g. "Ring vet to notify labour has begun. If no progress in x number hours, call again. If presentation looks wrong, call immediately." Doesn't necessarily have to be vet.
Are you all sorted with iodine for navel, castration rings if using them, tagging equipment?