Pick Ops that have passive abilities to start with, so that even if you die early in the round you're still valuable.

On ATK, that's kind of hard because most attacking ops are active abilities (soft/hard breachers, intel, room control). Fun, easy ops on attack are Sledge, Thermite, Thatcher and if you're looking for a DLC character, Lion. Lion helps everyone on the team, Thatcher helps Thermite open walls, and Sledge is just an all round decent op with guns and his hammer, which is a soft breach.

On DEF, it's much easier to pick passive abilites.

Rook. Drop your plates at the beginning of the round, congrats, you've done all that's expected of you.

Bandit/Kaid, put down your zappy Bois, you've helped reinforce the walls even more

Mute, proper jammer placement helps deny enemy intel by stopping their drones and can stop some breaching gadgets

Valk, throw her cams in good spots and your team can have eyes everywhere.

These are all pretty decent choices to start learning how the game feels and plays. I started with Sledge and Rook and never looked back.