For tracking contracts early on, for SaaS services you subscribe to and for legal contracts/documents use Excel (or similar spreadsheet) so people know what is there. You don't need an expensive fancy document management system to get started.
For tracking your other decisions. You have two different types of decisions that are critical to track IMO.
1: board/corporate decisions which need to be captured in meeting minutes and communicated out.
2: day too day decisions, for which a Word document that summarizes the key decisions or a wiki etc all work. I used to use a word document that linked to external documents for the supporting details on the decision. Today I like Quip for that type of thing, it is easier to deal with than a wiki and you can embed spreadsheets, graphs and all kinds of stuff in it, and linking to other docs is really simple. It is more or less a simple document management system at a fair price IMO. Quip just needs some structure put around how you use it and then it is awesome. Even policies and procedures are easy to track and manage in Quip.