Hit up exrx.net and look for weight lifting exercises that target back muscles specifically. For lower back (erector spinae), thing like weighted hyperextensions are going to target it specifically. I would suggest doing about 12 weeks where you start you back workout with basic compound lifts–deadlifts, squats, bent over barbell rows–and progress to more isolating lifts like hyperextensions at the end of the workout. I would suggest doing that 12 weeks with just 1-2 sets of 12-15 reps for each exercise, with rests of 30-90 seconds between sets and exercises; that will get your body ready to progress to more sets at higher weight and reduced repetitions. And yes, you need to change you workout every 3-4 months or so; that’s a basic principle of periodization. Changing your workout will mean more or fewer sets, with lower or higher numbers of repetitions, and different weight. In general, you’ll do a few sets of high repetitions with lower weights, and more sets of low repetitions with higher weights.

Also, don’t forget to do abdominals; those are important stabilizing your back.

For cardio, rowing is going to be your go-to choice for a strong back.

I would strongly suggest getting a personal trainer for a few sessions specifically to work on your form for squats, deadlifts, etc. If you do that though, be very, very firm with them that the only thing you want is direction on form; PTs will have a strong tendency to try and sign you up for months or years of training that may or may not help with your specific goals. Look for trainers with ACSM or NSCA certifications only (most other certs are barely worth the paper they’re printed on), ans a BS in kinesiology or exercise science.