I have a lot of information to backup, this includes accounts passwords, personal files, images and a lot more. So I was thining about backing up everything using a cloud service like Dropbox, Google Drive or OneDrive. My question is: Is this secure? Are these services always going to keep my files "forver"?
"The cloud" is marketing-speak for "other people's servers", so I will use that term instead.
A sensible precaution is to encrypt your files before you upload them to other people's servers, so that they can't see the content. But then you are responsible for remembering the decryption key. Lose your password and you lose your files. You might also miss out on some convenience features other people's servers provide, like viewing your files online, sharing them with others or automatic versioning.
Also, read the terms of service in regard to availability. They might not guarantee that their service will be available forever. The people might reserve the right to delete your files from their servers with a very short notice or even no warning at all. This is, again, usually worse with people who don't want any money from you. But even if they give you a written guarantee, that guarantee is worthless when they go bankrupt and have to sell the servers you have your data on. Taking legal actions against a bankrupt company is usually pointless.
However, when you believe you can trust these people, having your data on their servers can even have a security benefit. People who provide such services as their primary business model usually(!) have lots of know-how about how to properly secure a datacenter against logical and physical attacks. When you lack the know-how and resources for proper IT security precautions, letting someone else handle your files might not be a bad decision. But you should do your research about the security track record of the people whose servers you are storing your files on.