There are so many calculators on the market that it is imposible to give general instructions which will cover all possibilities. I will limit my suggestions to the two problems I have observed most often:

• Entering exponents: Entering scientific notation causes problems. Very large numbers like 3.00 x10^8 or 2.3 x 10^-6 should be entered using the EE key. You enter 3.00 EE 8 or 2.3 EE(-)6. Do not use x10 notation, it will get you into trouble.
• Entering angles: angles may be in degrees or radians or rads. Very often the problem is in degrees but the calculator is in radians mode or the opposite. Carefully check which mode your calculator is in.
• READ Your calculator's manual!