Try to build things that you need now.
For example, a classic mistake is to see a coastal site and say, "that's a great harbor" spot, only to find out it takes forever to get said harbor and you're stuck with a rubbish city. So if you need troops to defend, build troops to defend. Production is the most important thing early game; everything else is secondary.
Likewise, the tiles that immediately surround a city are the most important because tile expansion takes a long time. Spending 300 of your early gold to get your +3 campus spot looks cool, but it's also going to cost you the game. As for gold, be stingy. Do not spend it willy nilly because it does add up over time.
If you have a neighbor, exploit them. Sell your first luxury to them for some gold, and maybe steal their builders/settlers.
In line with the above, keep it simple. Build a campus in every city. Holy Sites if you want to go religion victory, Theater if you go culture. Don't build Industrial Zones, etc. Trade districts are great if you need gold, but don't go crazy and overbuild them if you don't need it.
You want to make the most out of cards. For example, if you have the card that boosts settler production, you want to do a lot of that when the card is active. Build under the influence of cards as much as you can.
Until you are more confident, you should mostly ignore most wonders unless you are confident you can chop it out with Magnus. There are not many wonders worth your while besides Colosseum. Most one city wonders aren't worth it beyond the memes-- Chichen Itza and Ruhr Valley are mostly traps. They look good, but are mistakes like 95% of the time if you're looking to win. I only build them because they look pretty.
Also, run lots of projects. If you want to be extra tryhard, then place your districts asap, but don't actually build them until later. This makes the production cost less.
If you're playing aggressive early on (and you should), try to avoid a dark age. The scout can be very useful there. If you're not being aggressive, it really doesn't matter.
Oh, and chop anything that gives production. Unless you're Maori who gains bonuses from those resources.
Professional Army (50% upgrade cost) is still overpowered. Thus, it's good to make a large army early on so you may upgrade it over time. In general, hard building units later in the game is not a good idea. Prebuilding is ideal.
Unless you're going for a cultural win or have scary neighbors, for the most part, I suggest ignoring diplomacy early game and simply sack your neighbors. I mean if you make a friend or two from joint warring that's cool and alliances are nice, but having more cities is better. I don't even waste gold sending delegations most of the time.
Beating emperor doesn't require any gamey/tryhard tactics, honestly. Just some fundamentals. However, unless you are really bored at King, I actually don't really think you're actually missing much. Same dumb AI after all, which is why I've never really appreciated difficulty settings for this series as a whole. You might also try playing King on more random maps or larger maps. Or with weak civs, or other special challenges.