Chisels can be used three ways ,  first with the heal against the wood ,  second with the  flat against it and third driving the point straight into the wood .
when you drive a chisel straight into wood the heal of the chisel will push the tip slightly towards the straight edge .
when you drive a chisel across a board with the heal towards the wood you supply a fulcrum to adjust the depth of the cut .
If you need to dig into the wood you can turn the heal out from the wood which you can also use to shave a surface flat which is only possible with a sharp chisel ,  so keep your chisels safe and sharp . You should have a sharpening stone ,  it is useful for many things in the house . see the link below on sharpening a chisel for more on sharpening stones .
if you are chiseling a mortise for a hinge  or plate or for a tennon ,  you should chip out a thin slot at the cross grain ends of the mortise .  This helps prevents the grain from splitting out past the end