The first step, as mentioned, is to determine the balanced chemical equation for the reaction. Once you've done that, there are generally three steps to a stoichiometry problem:

1) get to moles of something

2) use stoichiometry of equation to get to moles of something else

3) convert from moles to whatever unit you're looking for

One of the "tricks" is knowing how to convert. Dimensional analysis (using the units algebraically to help you solve the problem) is the easiest tool, if you're familiar with it and can effectively use it.

So, to look at your first problem:

1) how many moles of Mg do you have in 24 g of Mg?

2) using the number of moles of Mg, how many moles of MgO will you produce? (remember the balanced equation?)

3) using the number of moles of MgO, what is the mass of that many moles MgO?

There are other complications that can crop up, such as needing to convert to milligrams, etc., or using density to convert between mass and volume, but you still have these three basic steps.

Problems like #13, while perhaps common as exam questions, are quite trivial and not very common in real life (or even in a teaching lab), since we can't measure moles directly and they don't give you any directly useful information. However, it is a very important calculation to know, since it is the calculation in step #2 above.

Hope this helps,

Dick