Just a quick glance at CMS2019 Discussions suggests a lot of newcomers getting stuck early doors and giving up on what is a gem of a game with its main theme and a couple of extra sims thrown in that soon become recognisable to anyone who watches UK/US resto programmes on the old box.
Having played the game through once - although in theory it never ends - I've therefore taken the liberty to produce a few lighthearted pointers to the complete novice who wants to play the conventional game without any cheats or shortcuts. They represent my views only that I hope will allow first-time players to solve the little problems they might encounter and encourage them to soldier on to grease-monkey heaven.
Don't bother with it. Many of the skills learned have to be earned and not until after investing a good deal of time, effort and patience. The game is self-intuitive anyway so in my opinion just crack on and follow yer nose...
Investigate all workshop items. Some are purely ornamental, others are practical. This will be useful when you're stood there, scratching your behind, and wondering where that bloody engine has got to...
There are four types of order, look at them, decide if you're yet competent enough to tackle them, and prioritise according to your immediate aims and/or needs.
As you progress, orders will become more complicated with a multitude of jobs to perform. How you tackle them is up to you but remember that one of the most common complaints is forgetting to replace a relatively obscure part and having to start the job all over again.
My own preference now is to break multi-job orders into sections, fixing each in turn: for example, engine bay, gearbox, front and rear brakes and suspension, body- and interior- work. It means breaking off and visiting the shops more often but it's a lot less likely to result in frustration-induced mental instability...
Shopping. A pain when the missus insists you accompany her, an even bigger pain in this game because you can't slope off to the pub.
For simple one-job orders you can rely on memory a lot of the time, but for complex engine repairs the parts list can be a 'mare. There is an in-game worksheet but personally, I use a manual self-produced spreadsheet with a generic list of parts in the order of my preferred workflow and empty boxes to either tick or make notes (apologies, but I'm unable to upload the file).
In my opinion, shopping is the game's one major bugbear but at least you don't have to sit there enduring endless ooh, isn't that nice? and can i just try that pair on for a third time please...
You purchase upgrades with experience points that can take some time to earn and should therefore be used wisely. Look at the upgrades available and prioritise. Early on, there are a couple that will save you a lot of guesswork, needless engine dismantling, and playing upsy-daisy with the car-lifter. Others are not really necessary until you have to take them but, again, that's just my opinion
And that's it. There's bound to be something I've forgot but then again I'm old. Just remember that it's your game, you bought it, you experience it how you wish. I could be a complete anorak and sign off with "Happy Spannering" but I won't....