Play the Objectives
If you've played Battlefield games before, this should be a no-brainer. If you're new to the series, this is going to be extremely important.
You'll earn experience when you kill an enemy but your kill death ratio isn't that important in Battlefield 1. You'll want to take out enemies when you can but your primary focus should be on the in-game objectives. Completing these will net you huge experience gains and if you're lucky, a victory.
The objectives differ based on the type of map you're playing. In Conquest, you'll need to hold strategic points around the map. Capturing and defending these points will get you experience points. In Operations, you need to assault or defend two spots on the map. Successfully capturing or defending these sites will get you points.
Killing is just part of Battlefield 1. Get familiar with the objectives of each mode and work toward each. You'll level up faster if you do.