If you’re completely new i’d recommend;
xlimitz6, kihz, j0nny117 have current high level videos, bi weekly uploads usually, marco and zero subset to some extent have good videos. BAS1STA had good instructional videos, fun videos, etc.
ForgeLSdown, oelysiumx, oglegithw all have good videos but don’t really upload anymore. Bunch of other channels.
But basics for unsc, that’ll get you to 40 ts in 3s;
Build 3 pads, 5 hogs while the pads are building. Upgrade fortress as your pads finish, and put up your 500 reactor and then a depo.
While you’re doing this you should be picking up crates and scouting a rush. You won’t need to build more than 5 hogs usually, unless they apply early pressure with covy.
If they send their leader to your base, send your gunner to theirs, shoot pads, turrets, and any unit producing structures, while ramming anything they try to port in.
This will defend very basic rushes and harassment.
If they don’t rush, clear an expansion with your gunner, and build a base. Usually you’ll aim for 7-9 pads at max, with 3 reactors and 2 depos, so you want to expand when you can.
Either build your 750 on your firebase third, advance a reactor, or build it 4th on your expo. If they have an arbiter, you want to go 3 tank then canister, as canister is great at killing arbiters.
Once you do this, build 5-7 tanks, then powerturret. Push, harass, do whatever. But as unsc, you want your mainline production to be tanks, as tanks kill most units favorably, and are one of the few unsc units that can deal with raging arbiters.
Mid/late game, you can tech hawks, chain amp, odsts, spartans, various field armory techs, all are good and help support your army.
As covy, usually go temple 2nd or third, front turret if you scout their unsc going gunner, upgrade leader / build special units, then attack early. Usually just build enough to deny their buildings, force turrets, or force them to get gunner, then transition quickly into air. While attacking, upgrade 2 pads into citadel, then build a unit producing structure and upgrade the rest of your pads (99% of the time, you’ll go covy air in 3s because unsc counters most covy mainline units).
Use banshees to fly around and be annoying. Force wolves, then expo and switch to tech 3 hunters. Banshees are very good because if they attack you with counter units, you just boost.