You get special, unique perks for completing the major questlines. Questing is highly recommended.
Master your classes! You can master up to two classes and when you do, you keep the bonuses of the mastered class even when you switch to another one.
Bandits are your best bet for early levels. They usually come in small enough groups to be manageable and are almost always around your level or slightly higher.
The Whiterun/Riverwood area is best for starting out - northern areas will be significantly more difficult.
Grind! This is supposed to be an ARPG experience, so grinding the same few dungeons a few times will help you get stronger to take on higher level dungeons. It also gives you access to more loot which can make you significantly stronger.
If you feel like you don’t have enough perk points, try seeking out and killing bosses! They give you perk points when they die. It will be challenging but the extra points are well worth it.
If you don’t like your character’s perk point selections, you can sometimes find a potion that will allow you to respec your perk points available for purchase from alchemists. It’s expensive though, so be certain you want to do this!
Overworld enemies are going to be easier and in smaller groups than dungeons, but avoid the larger overworld locations when starting out: Silent Moons Camp, Fort Greymoor, etc. Delving into caves and dungeons is a good way to get surrounded by a group of 6-10 enemies.