1. Well. the best way to look at your first question is to write out the chemical equation in either word form or chemical form so that you can look for hints for the types of reactions that are occurring.
magnesium + oxygen gas --> magnesium oxide
Since this reaction is in the presence of oxygen and burns, we can jump to the conclusion that there is fire or heat produed. Right? Do you know what kind of reaction that is?
This reaction also shows that there are 2 reactants and 1 product ...you are making one thing out of two or more things. Do you know what type of reaction that is?
2. First, you need to think about what conservation is? If energy is conserved, can it be created or destroyed? Can it be changed into a different type of energy? Next, you need to think about what an endothermic reaction is... What does endo mean? What does therm mean? Finally, you can put your answers together...if energy can not be lost, and energy is going into this reaction, where could the energy possibly be going?
3. I can help you will calculating moles, molar mass, and kinds of mole problems....Is there a particular problem you would like help with as an example?