This is a very broad question, but I'll give it a shot:
Why do Airplanes need navigation?
Navigating an airplane is like driving a car with sole reliance on maps. Except you don't have any signs hanging in the air that might give you hints where to go, and in bad weather it is hard to determine your position by looking down to the ground. Thus a pilot needs other means of position determination.
Flying goes back over 100 years now; way before people even thought about GPS being possible, flights were already made by reference to navigation aids. A (by now) kind of old fashioned navigation aid is the NDB(Non-Directional Beacon) which in its day's was very helpful and was even used for approaches! This is now replaced by VORs (Very high frequency Omnidirectional Beacons). Building a VOR is quite a task, in the US there are many compared to other countries, yet not enough for the amount of traffic.
Why are there Navigational Fixes?
Cars use streets to get from A to B. Imagine all air traffic going straight from their departure to their destination airport, this would end in a huge chaos. Therefore there were "streets", so called airways, established from one VOR to the other. Traffic flies along these routes, just like cars drive along streets. But the VORs only are still not enough points on the map to navigate traffic. A simple curve would need multiple VORs to be described (Yes, there are arcs around VORs but that's something different). Thus waypoints were created. Before the time of GPS they could usually be identified by two radials from two different VORs. By now there are more waypoints out there that are described by coordinates only and can only be used with a GPS.
Where are navigational fixes placed?
Usually they are placed in a way so that they help out in coordinating traffic. You will most likely find more fixes around airports, as they offer possibilities to controllers to spread the traffic and, whats even more important, guide planes to the runway. There are so called STARs (Standard Terminal Arrival Route) which bundle traffic approaching the airport from different directions towards the same runways, followed by transitions which guide them via the IAF(Initial Approach Fix) to the final approach. All those procedures are described using the waypoints, VORs and other navigational aids.
Where would you get details about fixes?
To be honest, I don't know of any source that can be used for real world flying. I know of databases used for simulator flying, which are still up to date and probably contain almost exactly the same information. Those are called "AIRAC" (not to confuse with the real world AIRAC cycle though!) and can be downloaded or come with certain programs. From there you should be able to get the information you need (if you want to write a program using this information). Again, these are not for real world flying! If you are just curious about the waypoints around you, this website is one example of a source for these kind of things (clicking "World Lo"/"World Hi" will display these waypoints).
I hope this answers your questions, if not try to ask a bit more precise!