All warlock spell slots are the same level (PHB p.107, under the Spell Slots section). As they gain warlock levels, warlocks eventually reach their maximum of 4 spell slots (at the 17th level of warlock), and their maximum spell slot level of 5 (at the 9th level of warlock).
Do note that you can cast spells with slots that are higher level than the base spell. For example: hold person, a second level spell, can be cast with any spell slot of level 2 or higher. If you were to cast it with a higher level slot, it would even count as a spell of that higher level. Many spells even specify additional effects when you cast them at higher level, though not all do, and they don't need to have such a feature in order to be able to use a higher level slot.
In addition, the reason for the relatively limited number of slots is that warlocks regain their spell slots on a short rest as well as a long rest (unlike most other spellcasting classes).