Its going to be a week after signing a contract with a firm in Germany but the start date of the employment is not until 2+ months. The contract mentions nothing about how to terminate a contract prior to the start date. It does mention a statement saying "Neither party may give notice of ordinary termination before the start of the employment relationship" but it also states the employment relationship comes into effect if the employee signs the contract. So how can an employee terminate the current contract with the employer in this case.
Officially? You get an employment lawyer and have them figure it out.
Unofficially? You just call the company and ask them what paperwork they need. There is no way they can force you to show up and there is no fine you'd have to pay for not showing (assuming no such thing is explicitly mentioned in your contract). So stating you will definitely not show up for work in 2.5 months, it's in their best interest to get that contract dissolved without a hassle as soon as possible so they can look for a new hire. They might not be happy to hear you won't start there, but Germany is not like other job markets you may read up on here. If you don't want to work there, they will be happy to hear that from you now instead of 2 weeks into your contract when you are legally allowed to quit and have already cost them two weeks money and time.
So your best option is to contact them and ask them.
Without in depth knowledge of German contract law, I would say that usually in these cases you would just contact the company and just tell them that you no longer wish to take up the position. You don't have to go into specifics as to why, it's not their concern.
"Hi there, Unfortunately my personal circumstances have changed, such that I am no longer able to take up the position offered in your company"
Obviously - take care that you aren't burning any bridges you might need later. If you don't start the job, it's unlikely they'll offer you another position.
Do this as quick as possible. Two 1/2 months before your start date shouldn’t be a problem. 2 weeks before the start date, they would make you pay - because in Germany a contract is a contract (and because two week before starting causes them problems, so they think it only fair to cause you problems).
Avoid this in the future.
PS. "Make you pay" doesn't necessarily mean money. There are lots of ways. But if you have a contract and just don't turn up, that will cost the company money, so they may very well sue you for damages. They can also contact your next company, and then you have some explaining to do. Companies in Germany take this personally.