Welcome! you're going to want to avoid the word "tone" on this forum. Plenty of people here have will have advice on how to strengthen your lower back, or even how to cut body fat, but not many people are going to respond positively to "tone."
What you will want to do is Good mornings, light deadlifts (I dont know your workout experience), and some reverse hypers. Stuff like side planks and barbell rollouts are also good for strengthening your low back, but really getting a heavy diet of compound lifts is going to help all of your core muscles. Good luck!