Exercise Plan

From the US CDC

All adults should avoid inactivity. Some physical activity is better than none, and adults who participate in any amount of physical activity gain some health benefits.
For substantial health benefits, adults should do at least 150 minutes (2 hours and 30 minutes) a week of moderate-intensity, or 75 minutes (1 hour and 15 minutes) a week of vigorous-intensity aerobic activity, or an equivalence combination of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed in episodes of at least 10 minutes, and preferably, it should be spread throughout the week.
For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 minutes (5 hours) a week of moderate-intensity, or 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalence combination of moderate- and vigorous-intensity activity. Additional health benefits are gained by engaging in physical activity beyond this amount.
Adults also should do muscle-strengthening activities that are moderate- or high-intensity and involve all major muscle groups on 2 or more days a week, as these activities provide additional health benefits.


https://www.cdc.gov/cancer/dcpc/prevention/policies_practices/physical_activity/guidelines.htm

The amounts suggest provide a good minimum you should shoot for. Modify depending on your goals. You have the treadmill, rower, bike, and stepper (and I assume an outside option for walking/running) to pick from to get the aerobic activity.

You don't mention what you have in the way of resistance equipment other than barbells. For resistance work you want to concentrate on compound movements. These are moves that use several muscles/groups of muscles (such as squat, deadlift, bench press/push-up) as opposed to isolation movements (just use one muscle group) such as bicep curls.

If you have not been doing resistance work, you may want to start with bodyweight movements first (no external resistance).

Although many people have a specific goal in mind, to increase general fitness mixing strength training with cardio is a good approach. There are a lot of online workout communities that lay out plans (some are free, some require a subscription) or you could come up with your own!
Personally, I shoot for about 2-3 cardio sessions per week, 30-60 minutes each, and 3-4 strength training sessions per week, 20-40 minutes each. Some days I may do cardio and strength, and some days are just one or the other. On my “off” days I try to still do some light activity - walking, yoga, or stretching.
With strength training it’s also good to focus on major muscle groups. So leg exercises one session, arms the next.
If you are going to come up with your own program, start by learning a few exercises for upper body and a few for lower. Focus on doing a few sets of those until you have good form, then add in new exercises. You don’t have to do endless sessions with super heavy weights.