As a long time 1.e4 player and short time 1.c4 player, I'd like to add 1.d4 but there are so many defenses. I am considering the London system against things, while adding mainlines vs. all the various openings one by one. Or is the Catalan a better start?
If you want to start playing 1.d4 you should as first thing have a look at some theory about the Queen's gambit declined, as it is the most natural (and historical) way to play 1.d4: even if you will not end up adopting it in your repertoire, it will give you a taste of the major ideas behind the queen's pawn opening (minority attack usually against c5-c6, occupation of c-d files) and a sense of where the pieces belong.
After you do so you should start preparing for precise defences to play against (as in every opening although White has the first move, the order strongly depends on what Black plays next): in this respect you might have a look at any online database to find what is played most - but to spare the time, among all the most often played are:
- Slav defence
- Indian defences (at least King's Indian and Grünfeld)
Or is the Catalan a better start?
I myself do play the Catalan (when given the chance to do so) and therefore by all means I suggest you to look into it: it is however a heavy theoretical opening (from both sides) but it does provide White with a slight edge throughout the whole game (at best play).
I am considering the London system against things
The London system is somehow more "natural" to play and it is lately gaining a lot of respect at high level (Carlsen and Aronian above all play it very often): you may find quite a few GM games to take examples from.
In general, as any other opening in chess, you cannot have your way around without putting some study into a few specific lines; however, there is a lot of free content online that you may surf, especially the Saint Louis Chess Club channel contains a lot of opening videos suitable for any level (according to me very thorough).
As you are probably aware of,
d4 is a totally different territory. In one word, the amount of possible replies is lower and there are also fewer move-by-move trickery, but
d4 is much more prone to transpositions, which makes it so interesting from my point of view. You'll have to know them. Some lines of the KID, Grünfeld or Benko are also very sharp of course, so be prepared.
You should obviously start with the Queen's gambit and build a repertoire for the main lines.
If you have been a
c4 player already, I advise you to play the Reti, which will teach you power of transposition btw. As I've explained in another post, you can dodge sharp positions with it.
There are too much to say, so I'll advise you to see the 15 videos that Christof Sielecki did (aka Chessexplained) on
d4 repertoire for White. These are excellent, and will introduce you to the main d4 openings and ideas. Here is the introduction video to
d4 repertoire for White.
I recently switched from 1 e4 to d4 by following the repertoire book A Strategic Chess Opening Repertoire for White by John Watson. It recommends 1 d4, 2 c4, 3 Nc3 against everything, so the first three moves are easy ;-)
After a while, I experienced some pitfalls and I started playing other variations as well. For example, I don't particularly like his line against the Grünfeld (exchange, Bg5, Rc1). In fact, the antidote is mentioned in one of the sidelines as an interesting idea by Svidler.
So I'm not saying all of his choices are ideal, but this book will get you started. Also, it saves a lot of time, since you'll don't want to play the other stuff anymore (1 e4, 1 Nf3, 1 c4) :-)
I've personally been stuck on the same problem when I tried to learn d4 seriously. There are many solid systems for Black, with many reasonable looking moves on both sides, and move order or transposition problems. IMO, playing opening preparation is one of the best way to memorize it, so it's frustrating when the opponent plays a valid but totally different system.
My solution for that has been to learn d4 openings from the earliest, solid enough deviations up to the longest analysed variations. I started a month ago so I haven't progressed much, but up until now I've learned a lot about BDG (Blackmar-Diemer Gambit), and I'm starting to learn about Stonewall Attack, Richter-Veresov Attack and Colle System.
The point is that this way, I can steer the game into an opening I've prepared early on, which gives me more opportunities to gain experience in it while avoiding famous Black systems. In particular, BDG covers 1... d5, and Stonewall Attack covers basically everything else. Once I've gained enough experiences with these openings, I'll move on to others that go a few plys further, and so on, eventually covering the big openings.
The problem with that approach is that it's very slow, and commits me to learn openings that didn't really interest me in the first place, even though the openings themselves are nice to play. If you want to quickly learn a famous opening, the best is probably to buy a book talking about it, study the proposed lines and positions, and then play it online while adapting to other systems Black may choose.
If you'll excuse the self-promoting, I have started a Lichess study following the method I've described, which I update every few days with new variations from books, games (including my own), and mostly computer analysis. It's very empty right now though, with only a lot of material on the BDG, but you could also start a study of your own and fill it with what you've learned through games or exploring variations.
You could also consider the Queen's Gambit ("QGD") in addition to the Catalan.
One major difference with 1. d4 is that the pawn in question is supported by the White queen, whereas with 1. e4 the pawn is not supported. So you need to study how to adjust to that at the start of play.
Also, I have heard it said that 1. d4 tends to lead to more closed positions, although I don't know enough theory to explain why, or even if that is true overall. But it is also something to look into if you are considering adding 1. d4 to your repertoire. I do know from a bit of experience that many players from the black side are lulled into thinking 1. d4 is safer, but there are plenty of examples, including the QGD and the Catalan, where White can launch devastating attacks that come seemingly out of nowhere.
Also, if you haven't already read it, check out My System by Aron Nimzowitsch. His advice on chess fundamentals has become standard, but he also relied on a hypermodern chess style that was able to make good use of 1. d4.
d4 is the second best initial move because it frees a Bishop and the Queen while making e5 and c5 harder, but it does not stand on a winning line. Both you and your opponent will have many chances to make losing mistakes. Choice of opening rests with White, but after about 10 moves, both will have to deal with their opponent. You must play well to avoid losing. Your opponent must play poorly before you have any chance of victory. Pick an opening you like, and you may recognize your opponent's mistakes. Several following comments recommend following d-side openings in some order or another. Pick an order that is interesting and works for you. You will achieve your maximum playing strength because of who you are and how hard you work, not because you followed someone else's path. World champions accept worthwhile instruction, but they are not limited by it. Follow your path noting what others have accomplished, but keep remembering that your accomplishment must come from inside you.