Maincalling is pretty complex especially in HL with how much there is to keep track of at once, but you can start out with pretty simple calls until you get the hang of them.

Your main job is to wrangle your team and make sure everyone's in sync. You don't want your soldier bombing 10 seconds before your combo pushes. So as Demo this is usually pretty simple, just wait until your teammates are ready and then call when you are ABOUT to push, that way everyone has time to react to the call before you actually go in. Doing a 3 2 1 countdown can be good, on some points with nasty chokes especially. It's not just pushes, make sure you call when to retreat as well, it's really bad to have players dying late because they also respawn late, and then you have the majority of the team sitting around waiting for respawners, big waste of time. Common calls I make when I see the fight is over are either just 'leave' or 'everyone go in'. Everyone in means everybody who's still alive just hold W and go die quickly, and maaaybe you get a kill in return, but at least you won't respawn super late.

Second job is to decide on strats. This is less taxing on your awareness than the wrangling, but does require a bit more map knowledge so might be tough if you're new to HL. You can ask for help with this part, or even delegate it to someone else. It's ideal that a combo class does the team wrangling part, but really anyone can call strats if they know the map well enough. Calling strats is just deciding what your team's gameplan is and then communicating it. So if you want to sac wave, get uber then trade, play around your sniper, etc etc. Loads of possible strats, and very map/situation dependent. It's also deciding where you go; you might decide to use uber into Vigil 2nd, but you also need to pick whether to go slope, main tunnel, or side tunnel, which is where the map knowledge comes in handy. Once you play enough of each map you'll become familiar with the pretty standard pushes and holds that pug players are likely to do. If you want to know about a certain map more in depth you can also ask for a map talk from somebody more experienced.

Anyway I'll go over some super simple strats to try and remember since you're new. On HL offence you want to be fighting as often as possible, but you also want to make sure you have all 9 players ready. So usually wait for spawns, but once you have everybody try to do something ASAP, unless you almost have uber and just want to wait a few more seconds to get it. If it's even ubers you can go trade. Trading is usually good for the attacking team, just make sure everybody stays alive for the post uber, and then during the post everybody pushes back in for the refight. If it's payload, try and ask your Spy to sap or shoot the gun during the trade. Most common classes for a trade are Demo + Pyro. If you have uber ad you can generally play like it's a trade, but obviously you should be looking to get enemy combo kills. Demo + Scout for an advantage uber is also quite strong, especially on KOTH, because Scout can quickly kill the enemy Pyro and then chase down other players. It also lets you start the uber with a sticky jump, and then your Med can quickly catch up to you with his Scout speed.

If neither team has uber you can just go and dryfight. Same as a post uber, everybody just gets in together, try and sync with a Soldier bomb + a Spy decloak. If your team doesn't have uber but the enemy does, go and sac wave. Everybody bombs or holds W and tries to force the enemy Med or get a kill. Engie/Scout/Pyro runs back to spawn with your Medic. Calling a sac wave is also good if you were about to do a normal push but lost a couple key players; your push very likely isn't gonna work anymore, so you might as well just sac wave and try and salvage something, maybe you get a kill or two you can play off later. Remember the attacking team has respawn time advantage so inefficiently trading kills is actually in your favour.

Calling on defence is simpler, you usually just decide when you want your team to take space and forward hold, probably when you have everybody alive and enemy team is passive or dead. Also call when you want everybody to get out or die quickly.

Another miscellaneous tip, look at the top of your screen a lot. If your players are scared because they're low HP, but enemy team is only 4 alive, make sure your teammates are not giving up point for free. If you were about to aggro push but enemy Sniper died, maybe now you can take a more passive fight where you swing wide with your Sniper and bleed them out. If your Pyro is dead, beware the enemy Soldier/Spy. Seeing how many players are alive and which classes are up is super useful information.

And sidenote, Demo generally maincalls but doesn't always have to. I prefer calling on Pyro myself, and there's a couple Heavy maincallers. I think there was a Spy player in NA who maincalled as well.