I'd start with known tracks where there are plenty of vids (hotlap vids for example) for and start trying to mostly mirror these lines. Over time you'll get a feeling for the lines on other tracks of your own.
One Hour Magic Championship - Season 3 (Main), 4A, 4C, 5: Drivers Champion & Season 5: Teams Champion Formula E - Season 3: Drivers & Teams Champion Multi-Class Endurance Championship - Season 5: Overall Teams Champion One Make Endurance Championship - Season 3: Drivers Champion
Best tip i can give you on how i learned racing lines:
Get to know the GTAV map in general, multiple tracks will re-use the roads which will result having the same racing line mostly. Get together with other people and learn from those lines, see where you might win time or lose time and improve upon that.
And lastly, Race, race, race. Maybe it is just me but it took me quite a while to get some decent pace (Few months), And yet i am still not sanic fast like some others in this crew
When you do a race you've never done before, late apex on all corners on the first lap. It's a little bit slower but it makes sure that if there are any suprises around the corner, you can more easily adapt to it.