Casting most spells spends a spell slot of the spell’s level or higher, and you regain all your slots on a long rest. Casting a spell with a bigger slot often makes it stronger — that’s called upcasting. Cantrips never use slots; you can cast them at will.
How many slots you get depends on your caster type. Full casters (Bard, Cleric, Druid, Sorcerer, Wizard) advance fastest — a 1st-level slot at level 1 and their first 9th-level slot at character level 17. Half casters (Paladin, Ranger) start at level 2 and cap at 5th-level slots; third casters (Eldritch Knight, Arcane Trickster) start at level 3 and cap at 4th.
Warlocks are different: their Pact Magic gives a few slots that are always your highest level and come back on a short rest, which is why they cast fewer spells but refresh them far more often.
Every spell guide lists the spell’s level and the classes that get it, so you can see which slot it needs before you cast.