The key to lifting injury free is to keep the intensity manageable and the form correct. The big 4 lifts: squat, bench, deadlift, and overhead press are excellent staples that give you a lot of benefit for the work you do. However, if you are starting out you may want to work up to them. The posterior chain is important, and one of the first things we lose when we sit in chairs all the time.
I might start with this for a few weeks to build some base strength:
- Rounded back extensions. 4x8 at body weight. Lower your body so the back is rounded, and lead up with your shoulders. Try to work up to 5x10.
- Lat pull downs. 4x8 at whatever feels challenging, but you can still do with good form. Work up to 5x10.
- Lunges. 50 reps total (25 each leg). Start body weight, and when that feels really easy use dumbbells at your side.
- Incline press. 3x5 at a weight you can control for the whole movement. Bring the bar down so that it is parallel with your chin, and then back up. Bringing the bar lower won't make you that much stronger, and has the potential of hurting your shoulder. Be conservative and start light--even with just the bar.
This lets you build some foundational strength that will be necessary for performing the big 4. It also lets you do something that isn't quite as technical to get started. Once you feel confident with that, I would recommend moving on to something like Wendler 5-3-1. Your stated goal is to at least maintain your muscle, and bring back some of what you lost--not try to get as strong as possible as quickly as possible.
Wendler 5-3-1 only increases weight once a month, and keeps the weight manageable with weights you should be able to handle if you choose an easy starting point. It has the concept of AMRAP (As Many Reps As Possible) on the top set of the main exercise for the day, and that is AMRAP with good form. If your form starts to break down, stop the set.