Room Treaments - Where To Begin...

@oregon +1
Yes, The complete guide to high-end audio, it’s a great book.
Chapter 4: How to get the best sound from your room.

 

@mike_in_nc  Yes, that is the fundamental beginning.   I custom built a room which addressed bass (built-in activated carbon filters).   After your 3 recommended fixes, Shahki Hallographs (unless you have really fantastic soundstage and imaging speakers, then I use Synergistic Research HFTs (or use expensive and bulky quadradic diffusion paneling).

@gnoworyta   Let me help you cut through the muck here - - I've been exactly where you are at, and it can be overwhelming and confusing (see the dozens of different opinions here; plus your own quandry just not knowing where to start):

After two years of reading, studying, interacting with acoustic engineers, and then beginning to make plans for traps and diffusers I thought I needed, I was almost back at square one.  In the end, I was so glad I was PATIENT.  Don't be in a hurry.  Buy once, cry once - - save yourself a lot of time and effort (and unstudied purchases or wasted building of trap designs).   

You must, absolutely, measure your room first.   Room measurement will not only tell you which type of treatment you need, but also where to place your speakers (and your listening position as well).

Here is the microphone you need $79...that's all you need. Period.  
Acoustic Measurement Tools: UMIK-1 (minidsp.com)  

Then for REW (Room EQ Wizzard) tutorial, you can learn to hook up the microphone and run the free software.   There are 3 good tutorials for REW that will get you well on your way to treating your room, placing your speakers in the room, etc,   Note:  Ethan Winer at RealTraps will also help you with tutorial links or advice.  He is honest and won't sell you anything by pushing, etc.  

John Sayers' Recording Studio Design Forum • View topic - How to use REW to analyze the acoustics of your room... (johnlsayers.com)

Take the time to do this first:  despite all the shortcuts recommended in the comments here, the fact is, there are none.  "The Elephant is the Room"

Do this and you will know your room, and you can then research which treatments will mitigate the room problems.  You cannot build or buy traps until you know the problem and its source or location.  This is proper application of a particular treatment product in YOUR specific room by the analysis you've performed.  

Begin patiently by buying (or building) the particular treatment "panels" and don't buy all of them at once.  One step at a time with just a few panels is a simple and much cheaper (and less confusing) way to solve the problem and get excellent results.   In addition, you may find you cannot afford to buy or build all the panels you need; or, you may find you don't want the looks of too many panels.  In this case, you would know where to get the most benefit for your room for the least amount of money (or time and materials), and get the best looking room.  

This process is not that complicated:  just take it step-by-step, approach it systematically and methodically, and follow the science....and be patient.  Forget all the voices.  Just take charge of your room, learn along the way, take your time, and you'll be proud of your room and the results in the end.  Feel free to PM me if you have questions.

+1 on Jim Smith’s book Get Better Sound. 

In particular how to pick the best listening position but there is a ton of other useful info (most of what is discussed here).

I used to have a bunch of tube traps. Now nothing except an optimal listening position an Vandersteen 5a carbons with the 11 band equalizer tuned to the room. Much better looks and sound.

Now nothing except an optimal listening position an Vandersteen 5a carbons with the 11 band equalizer tuned to the room

An EQ??? Turn in your audiophile card immediately!! (Being sarcastic)