Since I am marking many first year exams here are a few tips. They are all basic but it is amazing and discouraging how few students do the following:
Clearly state any assumptions you make. Don't just write down equations. e.g, explicitly state, "Because of the Newtons second law ..." or "we neglect transfer of heat to the environment."
Keep track of units at each stage of the calculation. Don't just add them in at the end. All physical quantities DO have units. If you make a mistake it will often show up in getting the wrong units.
Clearly state the answer you obtain. Maybe draw a box around it. e.g, "The change in entropy of the gas is 9.3 J/K".
Try and be neat and set out your work clearly. If you make a mess just cross out the whole section and rewrite it. I see many students exam papers where I really have no idea what the student is doing. It is just a random collection of scribbles, equations, and numbers....
Think about the answers you get and whether they make physical sense. e.g. if you calculate the efficiency of the turbine is 150 per cent or that the car is travelling at 1600 metres per second you have probably made a mistake!