I've been learning about Cyber Security and how to protect myself online after reading some books on the subject and I want to know how much Information i've handed out to companies.
Depending on the applicable data privacy law in your country, you might have the right to request a company to send you all the personal information it collected about you. This usually works fine, though you have to send this request to each company you think is storing some data. Also, data privacy authorities often provide standard letter for this.
There are also some international legal frameworks for data protection, such as:
The key point for those national or international frameworks is: The data subject has the right to be informed when his personal data is being processed.
Now, you will also find different situations where it's hard or impossible to obtain this information. This includes companies that don't comply with data privacy laws, or information that is stored without the knowledge of the company (this can happen when a company purchase a large data set for marketing purposes, but only use part of this data set and don't look exactly at what it contains).
You can also perform simple search-engine searches based on your personal information and see what comes up.
Along with attempting to request the data, you can always read the privacy policies of the companies you are concerned with. The real issue though will be in (1) determining just how many companies have your data and (2) determining how much data they have of you.
If you are just looking for logins, newsletters, and such you could try using a tool that supposedly unsubscribes you from things by checking your inbox to help form the list of companies that have your data. Here is one service like that https://unroll.me/ but personally, I havent vetted it so be careful as so many of those things dont work as advertised...