Ask Game Masters

First, learn all the rules and figure out how the action economy works. Understand what things make a character or monster powerful. I'm not going to go into specifics because that covers the full chapter on combat rules plus a decent amount of monster stat blocks and the full character sheet of each of your PCs. Yes, reading is required, but so is comprehension.

Then, just like eerongal stressed, you need to know your party. If you have third level characters with between 16 and 30 HP each, you probably shouldn't have a monster that does 5d6+8 damage on an attack regardless of how few HP it has (that one is personal experience, DM thought me spending all of most combats bleeding out was "tough but fair" and I left that game after three sessions). Because instadowning a PC may seem balanced from an enemy point of view but that player is now a spectator and thinks your encounter sucks. Conversely the other extreme of a giant HP sponge that just chips away with one or two weak attacks per turn while the party surrounds it and pounds on it is just boring. If you learn how combat works, not just how to roll attack vs AC, you can balance between the extremes. Yes, this requires you as DM to do some homework.

A common basic suggestion is multiple weak enemies that individually go down quickly but present a threat through sheer numbers. The danger of this is the party rolls poorly in the first turn and the weenie swarm rolls well to hit the frontline tank with seven individually weak hits and now they're either down or definitely will be next round. Or the wizard gets high initiative and smokes them all with a single AoE spell because you had them all clumped up. An answer to both of these issues would be to spread the swarm both by approach and on their targets. If a PC can potentially die from being hit by four of a particular attack, don't target them with seven of those attacks in one round.

Another tip is legendary actions and lair actions. One basically takes a single big HP sponge and gives them extra attacks equal to multiple enemies. The other is some sort of environmental hazards like collapsing floors, falling rocks, poison gas clouds floating across the battlefield, preset traps, automated weapons, and anything else you can think of that isn't actually being done by the enemy themselves. Another common strategy is one "boss" that's actually fairly understrength against the party but supported by several weak mooks to use as meat shields and distractions; done right the group should deal just enough damage and take long enough to beat to feel dangerous and challenging but never be at risk of an actual TPK.

Again, you absolutely must know your party's strengths and weaknesses. Don't build to completely cancel out either, but make sure you aren't going to instantly wipe them and then plan around their advantages. If your PC is immune or resistant to poison, have one enemy that attacks with poison and several that don't...but have the venomous one attack someone else first and let the players feel clever by switching around.

Also don't fall into the lazy habit of one fight per day so the players can use all their coolest abilities every fight. Make them hold back a little because they don't know what else is waiting for them around the next corner. Again you have to balance for your party in particular, but a group of mooks that are easily defeatable coming fresh off a long rest suddenly becomes a lot scarier when everyone is below half health and the whole party has two spell slots between them. Again this takes practice and adaptation on your part. Plan ahead with encounters that can be adjusted with a couple extra (or fewer) of those backup mooks if the party did better or got more banged up than you expected in previous encounters. Maybe just don't use a boss monster's legendary actions if you need to dial back a bit, or give them some extra HP if you want them to last an extra round.

Last specific suggestion I have right now is that if you need to ask for this kind of advice you should absolutely not be running a mid or high level game at all until you can comfortably and consistently balance encounters for a low level party. Schools don't try teaching calculus to first graders for a good reason. You have to learn the basics first. High level PCs and monsters have more abilities, those abilities are more complex and can be used different ways, and the more complex a situation is the easier it is to fuck up. Interesting enough I also recommend against first level play because everyone is so fragile a run of bad rolls can get them killed by a couple really aggressive stray cats. I suggest level 3 as a starting point and slowly work your way up from there; PCs have a few special abilities to consider but not a lot and should be durable enough to take a few bumps. Low level combat shouldn't be long and drawn out, either, because four rounds gives you plenty of danger when your tank only has 26 HP. Conveniently the game designers gave us a tiered progression system that slowly and steadily introduces new and more complex elements as you adjust to the previous ones. Almost like they thought of this issue, imagine that.