There is a set level progression that for the most part bounces back and forth from the west to the east as you work your way up north. In other words the further north you go, the higher levels you'll see. The one thing that is an exception is the town in the southwest corner which is a late game area. Also one of the first things I wish I knew earlier was to click in the right stick on the map screen to lock it pointing north.
If at any point you feel stuck, exploring can lead to really strong items and tms right from the start. My first playthrough I didn't understand the geography well and thought I had to beat a team star base to go north, but you can just find ways around them. If you do go far to the north early, you will want to carry poke dolls to escape high level wild pokemon if you accidentally run into them.
You may have noticed already that poke centers are fast travel points, but all towers are fast travel points too and certain landmarks. Fighting optional trainers and talking to the pokemon league reps that hang out at poke centers gets you rewards - some of which are really great. One thing that is good to know is that trainers with black backgrounds to their text boxes have higher level pokemon compared to other trainers in that area. You usually want to save those battles for later.
