A lot of people get a mount for a smartphone, then use the TrailForks app. It's maybe the best database of GPS tracks of trails currently. You can view the trails on a topo map as well as use pre-existing routes, or generate your own route and follow that (that's hit and miss).
That's the inexpensive route. Issues with using a phone are 1) Damage to an expensive cell phone, 2) Battery life is not typically as good as a dedicated GPS unit and you may not want the phone battery draining in case you need it. 3) All GPS receivers can suffer from signal loss if riding in dense woods. Many mt. bikers add a speed sensor to get real distance measured on a ride, but you need to determine if a sensor you want will work with your phone, they pretty much all work with dedicated GPS units.
Or get a dedicated GPS unit. Wahoo's are very reliable, maps are Meh !., Garmin is the 800 lbs gorilla, makes great units, have good maps and screens to display, but can be buggy at times. I love my Garmin 1030, it's not cheap, but has a decent screen and works very well. I think a basic Wahoo Bolt is currently on sale for about $230. Some Garmins are on sale, The 1030 top unit is reduced to $400, the 520 Plus is $190.