@d0su remember that chess is something you learn over a lifetime (your "chess career"). Putting your repertoire into a box too much can deter innovation, ability to effectively understand unfamiliar positions, and maturation in general. So just remember that any and all openings in your repertoire can be changed at any time. I alternate quite a bit between 1.Nf3, 1.d4, and 1.e4 because they've all been my primary repertoire choices at some point. My ability to learn new openings is improved because I'm familiar with all sorts of pawn and piece structures. I can only give my recommendation that over your chess career you keep your mind open to the possibility of changing your opening repertoire based off your results, personal preference, or curiosity. You will likely learn a great deal more about chess in general than you will if you attempt to make your repertoire airtight.
So how should you start the game? I absolutely recommend 1.e4 because it's unlikely you'll get into a fully closed position. Black's options in the King's Indian Defense and the Slav are probably enough to deter you from enjoying 1.d4 as much as you'd like. At some point in your chess career you should learn how to play against these openings, but now might not be the time.
There are two branches of theory you should be aware of when starting with 1.e4. They're called the "open games" and the "semi-open games". The former refers to 1.e4 e5 (also called double king pawn openings) and the latter refers to asymmetrical king pawn openings such as 1.e4 e6, 1.e4 c6, 1.e4 c5, etc.
The big choice you should be making in the open games happens after 1.e4 e5 2.Nf3 Nc6. Your options are 3.Bc4 (the Italian) or 3.Bb5 (the Spanish). I would recommend you learn the Spanish -- the depth of material is appreciably larger, there are very few non-fighting drawish lines, and the material is very rich. As you become a stronger chessplayer, the Spanish will only continue to offer more variety and interesting lines. The Italian, on the other hand, may score you some nice wins at your current level but will only become a more dull weapon as your rating grows; players with the black pieces that know only a minimal amount of theory are generally capable of holding equality against 3.Bc4. If you don't believe me, look at the games of some of your favorite high-level players (hopefully since 1950 when more scientific methods of chess improvement were employed). You will rarely see 3.Bc4.
There is a third option if the Spanish doesn't suit you -- 3.d4 (the Scotch opening). Like the Italian, the Scotch tends to outlive its usefulness when black understands the theory and can equalize with simple forced opening sequences. It tends to offer more rich piece play than the Italian though. So if you choose not to play the Spanish, the Scotch opening should probably be your second string.
And then there are the semi-open games, arguably the most interesting openings in chess. Let's start with the most popular and work our way down:
1.e4 c5 (the Sicilian) is a popular way to meet king pawn openings. It's the main alternative to 1...e5. The complexity of this opening tends to be greater than that of any other opening. There are two main options for white here: the open Sicilian (the main line) with 2.Nf3 and 3.d4 (against black responses of 2...e6, 2...d6, 2...Nc6), and anti-Sicilians.
You'll hear a common argument voiced that because of the breadth of responses you have to learn to the Sicilian, you should play an anti-Sicilian repertoire. There are reasonable arguments to be made here: some players claim they don't have the time to learn lots of variations, some like beating up on people by taking them out of their opening preparation, while others think they're just the objectively best way to meet the Sicilian. If you want to take the lazy approach, go with an anti-Sicilian repertoire. But be forewarned that by eschewing longer theoretical debates with your opponent, you're likely to learn a lot less about chess. In the long-term you're pigeonholing your understanding of the most educational opening to "easy lines". It's a lot harder to learn the open Sicilian after you've played a lot of anti-Sicilians; much like eating junk food will abate your hunger, but not nourish you over your lifetime.
There are common structures for white in the open Sicilian. You can choose a modest Be2 setup with Kingside castling if you like a more positional game, or go for a Be3/f3/Qd2 setup with queenside castling if you like to attack/defend an opposite sides castling game. There are relevant and decent sidelines in the open Sicilian with g3 setups, early f4 setups, and Bc4 setups. Each branch of theory is specific to black's choice of Sicilian (2...e6, 2...d6, or 2...Nc6 Sicilian), so there's a decent amount to learn. Fortunately there are books like 'Experts Against the Sicilian' that give you a repertoire choice for each of these options, so learning isn't as daunting.
Another common choice for black in the semi-open games is the French, 1.e4 e6. After 2.d4 d5 I would recommend you play 3.Nc3. The positions will sometimes be similar to what you experience on the black side of the Scandinavian. The move 3.Nc3 is popular at all levels and can lead to more closed, or more open positions depending on black's choice to exchange or decline the e4-pawn. This is probably the only opening where you can't do much to avoid a somewhat more closed position unless you want to plat the exchange variation (3.exd5), which is not considered a very good fighting option for white and is instead drawish and symmetrical.
The Caro-Kann is next on the list. Similar to the French, you can play 1.e4 c6 2.d4 d5 3.Nc3 against the Caro-Kann and enjoy a semi-open position that may also be similar to the Scandinavian. The pawns in the center don't usually move around a lot (though they're not locked together) so you're afforded good piece play here. 3.Nc3 is popular at all levels, up to the top GM level.
And then there are the ...g6 defenses black can play against 1.e4. The Pirc (1...d6) and the Modern (1...g6) are generic, catch-all black systems that can be played against virtually anything. I recommend you choose whatever line you're most comfortable with against these defenses, keeping in mind that whatever system you use with 1.e4 you can probably use if you ever choose to play 1.d4, 1.c4, or 1.Nf3 as your opening moves. I like taking space in the center against these openings with c4, d4, e4 in traditional fashion, but you may prefer to keep the pawn on c2 in order to facilitate a less closed position.
A closing thought on your move 1.Nc3 -- don't play it. While not objectively a terrible opening, it usually transposes to another opening, which means you can find yourself studying 1.d4 and 1.e4 systems simultaneously. The 1.e4 systems with an early Nc3 are typically more tame and symmetrical than the Spanish or Scotch (the former keeps the c3-square ready for a pawn to support the central advance d4, while the latter can transpose in certain lines).
Some lines to summarize my recommendations:
1.e4 e5 2.Nf3 Nc6 3.Bb5 or 3.d4
1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.d4 or 5.Nc3
1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4
1.e4 e6 2.d4 d5 3.Nc3
1.e4 c6 2.d4 d5 3.Nc3
1.e4 d6 2.d4 Nf6 3.Nc3
1.e4 Nf6 2.e5 Nd5 3.d4 d6 4.Nf3 Bg4 5.Be2
