I need a really good excuse to tell a company (which I already signed a contract with) that I'm not going to start working with them

"Sorry I died yesterday..."

I took an offer from "insert company" for 35% more then here, I start on Monday.

@C0D4 now i do 50%, last time i did 30% they accepted haha.

“I know we signed a contract already, but I decided against ist afterwards, I just have the gut feeling you will suck ass. Thanks for your understand”

First check if the contract or any laws can make you owe them something for canceling the contract.

Any decent company would most likely not enforce you to come to work if you want to get out, you would not really be a contribution.

But if they have taken on costs they might want reimbursement or similar.

But it could also just be that they can withhold pay if you quite without due notice, which is irrelevant in your case ;)

As for good excuse, if the truth is not good enough I think it can be hard to fabricate a good one. It better be fool prof so they do not find out, that would be way worse than a bad excuse.

Just tell them. Better that than they find out on the first day when you just don't turn up.

Call them, be honest and wait for their reaction.
Normally that's not an issue. In my country even if the contract says you owe them. It's bullshit. Because they would owe you your monthly payment.

Be honest and see what will happen. No need to be stressed or struggle before you know the outcome.

If you have to go to the lawyer then you also be only stressed or struggled when he says you are fucked. Don't google law or medication. You can google both, but don't think you are an exoert by 10 min google.