Generally speaking there's 4 types of skills that are relevant to you right now
Speed skills, Acceleration skills (These two are yellow), Recovery skills (Usually blue), and Flat stat increases (Green)
During your training career, you can obtain skill hints (red exclamation mark from your supports) that you can proc once per turn. These can give you a skill hint that will discount a random skill from card's available pool, up to a maximum of a level 5 hint. Racing will occasionally also drop a hint relevant to that specific race.
Buying a skill during training will remove that skill from being dropped from future training hints, and will lock you out of gaining further discounts for that skill. This can be a good thing, but that is not within the scope of this short explainer. In short, buying skills will lock you out of getting more discounts for that skill, which means you usually want to hold out on buying skills too early.

However
Buying skills can help you win your races, so if you are struggling as a new player it can be quite helpful.
That being said, not all skills are made equal. If you're looking to simply win your career races, avoid buying any green skills.
Look for strategy locked or general speed skills that say things like "Increase speed/velocity on xyz".
Avoid speed skills with restrictions like the plague for now in career.

Also do not buy any skills that say "Acceleration" unless it is specifically "Shooting for Victory" or "Red Shift/LP1211-M" (both inherited ults) for now, and especially avoid anything that says "Randomly .... increase acceleration"

Buy gold blue skills that don't say "when boxed in" if you're short on stamina for a longer distance race you have coming up, otherwise it's largely a waste.

Any other skills not mentioned will not be useful