A simple question

[el minster]

When should a sorcerer use his/her spells?

[OrbanSirgen]

When using the spell is a better alternative than not doing so... It is completely situational...

[Kelb_Panthera]

At low levels, you want to be sparing with them. If it looks like the rest of the party will have things in hand without you casting a spell, grab your plinker (sling, crossbow, wand of magic missle, etc) and start plinkin'. If a foe or obstacle looks impassable without magical aid, fire away with those bad-boys.

By mid level you should have enough resources together that you can pop off at least one or two in each encounter and still have a few for out of combat situations.

At high level the magic should flow from you like water from a spigot. Fire away.

[el minster]

but how to use non-max level spells?

[StevenC21]

Those can be used for really any effect that isn't DC-dependent, like Rays, or player buffs, or just general utility spells.

[Biggus]

but how to use non-max level spells?

Have a look at this thread: https://forums.giantitp.com/showthread.php?587773-Good-low-level-spells-for-high-caster-level&highlight=low+level+spells+high+level+casters