It's tough to say because it depends on how it is where one lives. Due to the nature of the virus volunteering is hard. Where I live people can donate money to support the cause (Our town's mayor spent the cash in the budget for Easter lights and decorations). Some tailors have started producing N95 masks. Hotel owners provide free meals and accommodation to hospital staff and other people at need.
What I've done is help senior citizens with small tasks, I have a designated day in the week for grocery shopping and I shop for everyone who needs it as well. I also help them with paying bills, if there are any. Older folk, where I live, do not know how to operate internet payments and any help in that regard is very well received.
Something as small as a few kind words go a far way. My mother, who is a nurse, works for a private practitioner(who thankfully closed down before the epidemic) and she helps older people by reminding them that their appointments will be kept after all this ends, should they desire so, and calming them in general, with advice. My mom is a people's person and highly skilled in her work.
The people most vulnerable are hospital staff, elderly people and the ones with pre-existing conditions. Sadly, there is not much we can do to prevent over-exhaustion of hospital staff. As for elderly people, doing whatever we can to keep them safe is our best bet.
Also, never forget to keep a positive mindset. This sounds cliché, I know, but it's invaluable.