Bought stock several years ago for $5,000, worth only $100 today. Would like to sell and write of the loss. Can I do that?

sell before 12/31/2020 you can take the loss.  you may want to find a broker, if possible, that charges low fees. 

You would have a long-term capital loss of $4900.  If you have no other capital gains, you can deduct $3000 of the loss in 2020, and the remaining $1900 loss will be carried forward to 2021.  

If you have other stocks with gains that you are thinking of selling, you can use the loss to offset the tax on those gains, assuming you complete the sales by 12/31/2020.