A lot of this is from Aiming a Spell, but, in short, you should determine if the effect's a burst, spread or emanation; then determine the effect's shape (e.g. cone, cylinder, line, sphere); then mark up the battlefield. That's because
Most Areas of Effect Stop at Obstructions...
The key word is most. An area of effect--like a burst or an emanation--stops when it hits things:
A burst spell affects whatever it catches in its area, including creatures that you can't see. It can't affect creatures with total cover from its point of origin (in other words, its effects don't extend around corners). The default shape for a burst effect is a sphere, but some burst spells are specifically described as cone-shaped. a burst's area defines how far from the point of origin the spell's effect extends.
An emanation spell functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell. Most emanations are cones or spheres.
...But There Are Exceptions
Spreads don't stop at obstructions. Instead a spread
spread[s] out from a point of origin, which must be a grid intersection. The effect can extend around corners and into areas that you can't see. Figure distance by actual distance traveled, taking into account turns the spell effect takes. When determining distance for spread effects, count around walls, not through them. As with movement, do not trace diagonals across corners. You must designate the point of origin for such an effect, but you need not have line of effect (see below) to all portions of the effect.
Also, cylinders don't stop at obstructions. Instead a cylinder has a
point of origin. This point is the center of a horizontal circle, and the spell shoots down from the circle, filling a cylinder. A cylinder-shaped spell ignores any obstructions within its area.
Emphasis, in all cases, mine.