In SAO:B leveling up is based on grinding to use better weapons, armor, and other things. Leveling takes a long time in SAO:B since there are no quests to help you get experience, its basically one big grind. Some noticeable points in leveling is there are some level gaps in using items, which means a very large amount of grinding. One notable level gap between items are level 25 to level 29.
Naturally, for beginners, start killing boars, once your level 5, get a Blade of Grass, or a Knight Blade, after that, start killing the wolves, then, start farming the Dire Wolf. Once you are level 8, use Skydusters and go to the floor 2 boss, kill it and go to Floor 3. From here, use Samurai's Platemail and dual Skydusters to battle the mini-bosses, once you get to the Floor 3 dungeon, kill the Jungle Golems and finally defeat the boss to floor 4. Floor 4 is the best level for XP.
There are some good places to grind in SBO. There is the boss room on floor 3, the boss room on floor 2, but the most popular farming place is on floor 4. On floor 4 is a place with a lot of frozen guardians. These mobs give 25 exp, but people with double exp would get 50, which makes leveling a lot faster for some people. The leveling areas though are all personal preference, as a person could like leveling on floor 5 killing centaur defenders.
Some good tips for leveling is that if you didn't know already, you need to do at least 25%-30% to a mob in order to get exp, col, and drops from it. So doing damage is very important. Also, don't level at places where you can barely kill the mobs without help, as it's a waste of time. Lastly, don't grind levels for hours upon hours on end. Take breaks so you don't get bored of grinding over and over again.
P.S. If you want, leave the game after you level. If SAO:B shuts down while you're grinding, you may lose a large amount of experience. For me, I was level 35, and then the game shut down. I lost 2 levels, so take this advice to heart.