Hi Doug, Thank you for your great question! Your aim, as you mentioned is to stimulate bone and since you have to continually challenge yourself you will want to adjust the weight as you get stronger. As a rule, if you have never participated in strength training it is wise to follow the following guidelines:
Build to 2 – 3 sets of 15 repetitions
Do not compromise exercise form or posture through your repetitions.

Once you successfully achieve the above for 3 consecutive workouts you are ready to increase your resistance (weights for exercises that require external loading and a more challenging gravitational position for exercises that uses gravity i.e a lower step for push-ups) or increase your cadence (the speed at which the exercise is performed).

You will increase the load enough that you fatigue within an 8 – 12 repetition set. Do a minimum of two sets. When you strength train you want to push yourself to do as many repetitions as you can to fatigue while maintaining good posture and good exercise form. As you get stronger you will find that you slowly creep back towards 15 repetitions if you can reach more than 12 reps on consecutive workouts and that will be your cue to increase your load again.

I hope you enjoy the process. I know you will enjoy the outcome!!
Wishing you the health for many years of strengthening.
Margaret