Most has been covered already. Always note the tire to tube position when removing from rim so you can look at the specific tire area when you find the leak in the tube.
If you do that, sometimes you need to press against the outside of the tire so you can expose a wire on the inside. Wire sometimes withdraws into the rubber and only pokes out to puncture the tube when the pressure of the road drives it inward.
Also check the valve. A loose Schrader will leak.
Lastly, you can add some stans fluid to the tube if you get stumped. I had a persistent slow leak a while ago and doing this solved the problem. Pour some in and go for a good ride to swish it around.