^ What gurmies said. Read the question carefully and know what you're being asked to find. Learning the 5 basic formulae required in HSC chemistry is also a good idea.
n=c.V (moles = concentration x volume)
n=m/M (moles = mass divided by molar mass)
n=V/Vm (moles = volume divided by molar volume)
n= delta H/delta Hm (moles = amount of heat released or absorbed divided by amount of heat released or absorbed by one mole of the substance)
n = N/NA (moles = sample number of units divided by number of units in one mole/Avogadro's number/6.022x10^23)
Knowing these simple formulas means that you easily manipulate them as required in questions requiring calculations. Make sure you know them really well. (Can't stress the importance of this enough.)
Also, as a rule of thumb, I tend to write the equation for the reaction involved, and write the given data beneath each reactant/product. (For example, if they give you the mass of a substance, or the volume of a gas produced.) Then, I'd write the missing data needed. (Eg. If you're trying to work out the concentration: "c=?") You don't need to do this, but it's useful because it saves time and gives you an idea of what you're looking for.
Other than that, there's no short cuts with this. You just need to practice, practice, practice and do so consistently throughout the year. Past papers are a good source of practice questions, and gives you an idea of the type of calculations required in the HSC. Alternatively, try the Chemistry 2 textbook by Jacaranda... It often talks you through calculations, step by step.
