Before anything else, understand that if you want to use a VPN you should be paying for it. Free VPNs are either selling your browsing data in aggregated form to researchers and marketers, or giving you a paltry amount of data transfer every month. Either way, a basic rule of thumb is that a free VPN will not protect your privacy in any meaningful way.
Some VPN services provide a free trial, so take advantage of it. Make sure you are happy with what you signed up for, and take advantage of money-back guarantees if you're not. This is actually why we also recommend starting out with a short-term subscription—a week or a month—to really make sure you are happy. KeepSolid VPN Unlimited offers a one-week Vacation subscription, for example. Yes, you may get a discount by signing up for a year, but that's more money at stake should you realize the service doesn't meet your performance needs.
VPN services, while tremendously helpful, are not foolproof. There's no magic bullet (or magic armor) when it comes to security. A determined adversary can almost always breach your defenses in one way or another. Using a VPN can't help if you unwisely download ransomware on a visit to the Dark Web, or if you are tricked into giving up your data to a phishing attack.