How do you learn new tracks?

I would do 5 laps of an average 2.5mi course to get a bearing of it. Drive it first either in your best class to race, the intended class for the race, or the slowest to get a feel for how the race will typically go. I would use whatever car you are best in given the class lest the race is locked to a specific vehicle. Also if the race is meant to be a stock race do it like that. The host will normally be the creator in that situation or someone who enjoys the creators race as the creator indented for it to be. As far as lines go, you will have to be adventurous and just practice different ones.

Get into a race with someone else who knows the track and try to copy their lines it'll be a lot faster than doing it on your own.

Haha that's how my car always looks within a couple of minutes trying a new track. Thanks everyone for the advice I'll be putting it to use soon. 

Once ive done a lap, i have a generalization for braking points, then each lap after, i push each corner a little harder than the one before to get the smoothest line possible. Under most scenarios, it takes me about 2-3 laps to get thr hang of a track, but sometimes my second lap on the track is my fastest.

First i have a glimpse at the preview map. If it's not too long, I try to memoize the outstanding features (Hairpins, chicanes, general flow stuff).

When actually racing, I'm more on the conservative driving side which means I'm not literally lurking around but when in doubt leaving some space and then increasing the speed in a consecutive lap.