Making the transition from road to trail running isn’t that difficult. If you’re a beginner to trail running I’d recommend joining your local running club, it’s an easy way to be introduced to some of your local trails in the safety of a group.
Kit is another important feature, trail running requires a shoe with a little more grip than a road shoe. A great entry-level shoe would be the Salomon Speedcross 3, which I use a lot as it’s okay to run on the road when I’m in-between trails. If you’re planning on heading out for a longer run it’s a good idea to have some essential items with you as you don’t want to be caught out, a map and compass for navigating, money, mobile phone, full waterproof body cover, hat and gloves, and you may need some food or water with you depending on the length of your run.