Simply play through the first 3 - 5 main quest missions (find and select them in the in game menu) OR explore and do a few side quests as soon as exploration becomes available. Level 5 should come within the the first 2 hours of playthrough maximum.
If you are having trouble grasping or understanding how to even travel to different locations or manage your character, try watching a guide on YouTube or just take the time to learn and pace yourself.
This is the most easy achievement in the game but the game itself can be overbearing and seem tedious at first.