I've played a 5e warlock quite a bit, so I'll just amplify a little on what Unseen has said.
A 5e Warlock has hewn close to the 3e version, in terms of how it relates to party role. A warlock has a small bag of tricks they can use with great frequency. Your invocations are often at-will, for example. And you can recharge your spells on a short rest...which makes a big difference as far as how often you can use them. And since you always cast at your max level, it makes even low-level spells potentially worthwhile...check the spell entries for specific ones that have extra effects when cast at a higher level slot. Pick those preferentially. You lose nothing you won't get back quickly.
Wizards and clerics and the like can 'nova' more spells in a single encounter than you, but by the end of the day they'll be down to cantrips while you can just sit tight for an hour or so and have all your goodies back. Your resource management is all in the moment, where everyone else has to look at the long term. This means you'll be flinging 3rd level spells in every fight...which makes warlocks holy (unholy?) terrors, if they pick spells that take advantage of it.
Spells to avoid are ones that don't scale with level, ones that have excessively circumstantial usefulness, and ones that can be easily replicated by invocation or magic item.
For item picks, check out various magic Wands, which can supplement your spellcasting ability, and the Rod of the Pact Keeper, which boosts your spell attack bonus AND your save DC.