If your chosen VPN provides a free option, of course, download it and install the software to get a feel for it.
However, be aware that you'll be getting a restricted version, possibly with bandwidth or data limits, or you might be blocked from streaming sites.
If you want to test those elements of a VPN, you'll want to sign up for a month to get used to the service. If you don't like it, most providers offer at least a 30-day money-back guarantee, and if you do like it, you can then sign up for longer and save a whole load of money.