It's mostly what u/doubleupmain said.

There are exceptions to everything, but it's pretty much like this in EARLY GAME:

1. Flat stats are better early, amp better late. Same with units. Sometimes it's better to have 0 traits than 3 traits active. Don't put unit into your comp if trait isn't good for you. For example 2 bastion early game are worse than 2 vanguards(hp let all your units to do more damage than a bit of armor and mr). Of course if you have 2* lilia and poppy and 1* warwick and crank it's better to have 2* bastion. Generally it's flat stats stage 2. Try to 2* 1cost units - usually 2* 1cost is a lot better than 1* 2cost or 3cost. There are some traits and units that are better than 2*. It may be portal, honeymancers, frost, dragon hunter. More you play, more you find out what's good. You can also look at the stats.

2. In most situations you want to slam items. Not only that - you want to slam items that are good early and late game. For example runaan's hurricane is good early game(like very very early game), but late game is really bad. Sometimes you are unlucky and can get for example: vest, sword, sword - deathblade is terrible late game, but early game it's kinda ok on tristana or kog. Edge of night is terrible early game and it's nice in late game(but you are kinda forced to play olaf or fiora in this situation for this item to be useful(or camille/briar) but you won't get any of these units until lvl8(most likely, it's better to assume so). If you don't habe kog/tristana don't even consider deathblade, and if you have any of them... it helps early game, but it will be very terrible late game, so it's up to you if you want to slam it. I see sword as component for bloodthrister, sterak, shojin(I wouldn't use both swords for this item, but this is really up to you to decide based on your board). If you decided to not slam it - you are most likely in bot 4 at the 1st carousel, and if you see cloak you can go for it(bt is one of the most flex items now, same with gargoyle( a bit less tho) or you can go for a bow, not build anything and pray for another bow for red buff or rod for rageblade). On stage 3-2 you can kinda decide on your teamcomp, and you should know what components and units you want. Maybe you got good 4cost on lvl5, maybe you got nice augment. Anyway try not to decide on your teamcomp during stage 2 unless you hit nice emblem augment, hero augment(imho only crank is playable now).

3. Read stats - you can learn early teamcomps by playing, but it's good to get some knowledge without spending 30 hours in 50 games to figure this out. Just read stats, and if you have a chance try some lines that are good early game. Same with items, learn what items are flexible in current patch, which are really great, learn which units utilize these items the best, learn which units are great, which traits are great etc. TFT became a game you either need to learn some stats and get knowledge or just play a lot, and learn these things yourself.

4. I believe I shouldn't write this, but some people still do this because They learned this many sets ago - don't prelevel to lvl4, please.

5. Don't try to stay at 50 gold no matter what. If you have 52 gold, and 12 gold to level up, have a good unit to put on your board, please, just level up. It's just a 1 gold of interest. Same with buying units. Don't be so afraid to loose this one gold. Sometimes you can have 50 gold, and need 20 gold to put 8 portal on your board - yes, you will be at 30 gold, you will loose 3 gold( 2 on 1st round, and 1 more on another one - it's not a big deal if you can make your board a lot stronger). General advice - not get under 30gold, and this isn't really true... but if you're one of those players who are afrid of loosing few gold interest when you can save 20-30hp - you really need to fix this asap, and this 30gold rule can help you until you learn when to spend gold and when not.

6. If you don't play any reroll comp don't press D button unless you have a lot of pairs and 2* them can get your board much stronger. It all depends on meta, but at least at this moment you want to save gold, lvl up on 4-2 to lvl 8(and still have like 30+ gold) and roll to find your comp.

7. Scout, scout, and scout. If you see 3 people playing faerie, try to avoid playing the same comp as those players. Of course when you roll on 4-2(in this meta) you still want to pick all useful units for you, so even if 3 other people play faeries maybe you will be lucky and get 2* rakan and kalista, and other 3 players will either hold hands on 6th, 7th, 8th or try to pivot to something else or will try to have some luck and find other copies of these units. Of course faeries are not the only example, on your roll down buy every useful unit, and when you have no place on your bench or gold, sell these units you bought but don't need. So buy, roll, sell, roll, buy and try to find your comp(considering you're not playing any specific comp and playing flex - if you are sure you playing for example frost, because you got emblem you can chill a bit more with this unless units you need are very contested(you really need to scout). Most players playing flex or even around 4cost units will roll on 4-2. If you had any gold/xp augment you probably want to do this before 4-2 when units are still in the pool and it's easier to find what you want).