If you are not used to exercise the recommendation is to start with a relatively short session maybe 10 minutes at a time (once or twice a day), then work up from there until you are doing 1/2 hour maybe.

Simple exercises like push-ups, squats, sit-ups, etc. are good enough unless and until you get "serious" about it (so no need to buy machines like exercise bikes for now!). As often is the case, a simple kitchen timer is all you would need.

There is plenty of information on the web about the exercises you can do and of course tons of professional advice as well (so that you can double-check this advice, of course).

VV