Do not know how to make the shower get hot

Check the water heater to see if the outlet shut-off valve is closed slightly. Maybe there is a valve just for that particular bathroom in the attic.

If neither of these are the case then it is time to check the washers and o-rings inside of the valve body. THis is accomplished by turning off the water supply where it enters the house, removing the handles, loosening the packing nut, pulling stem out and changing the washers. The replacement is just the opposite.

Hope this helps.