This is a hard question to answer since it's pretty vague, but I'll try to give the most general help.
Depending on what the turn is, you basically try to do the out-in-out approach - start out wide, enter into the apex without hitting the corner, then exit wide.
Unfortunately it doesn't apply to all corners, such as a chicane. You just have to find the best line for that corner that gives you the best advantage for the next upcoming corner.
As for where to brake or go on-throttle, sometimes you can coast it into the corner and roll into it, then accelerate out, but you would probably find that slower. That only applies to a large radius IMO, but its a different story for 1/8s - in a corner where 1/10 would brake soft, the 1/8 would still be going full throttle, let off and dive straight in! I know this at a track I sometimes go to, absolutely wicked.
So all in all, just find the best/fastest line for you, and improve yourself until you find where you lack.