Everyone else has given great answers, but just to summarise:
1. "Look for trouble" in conquered territories to fight Goons which will level you up and give you the opportunity to learn skills (see #2)
2. Learn new skills! Put a girl in the frontline and MC in the support position and then use the girl's unique skills. MC has a random chance to learn that skill every time the girl uses it.
3. Pay attention to the "Scout" text and listen to Jessica's advice! Some enemies are weak to physical damage, others to seduction, and some require a more defensive playstyle. Jessica will give you hints about all of this.
4. Remember that all enemy skills can be negated or countered. Noelle about to throw an Impale your way? Block it! No damage = no chance to kill.
5. Make food! You start out with a recipe for HP-restoring food and you can get a WP-restoring recipe from Hillside Development. Each one restores 50% of your maximum, so level up and make use of the percentage-based healing!
6. Don't be afraid to switch someone else in as a frontline fighter. MC has the greatest potential and highest utility, but sometimes a different character is better as a frontline fighter in specific battles.
7. Pay attention to the girls' support abilities! Going up against a strong physical fighter? Bring a girl that buffs your DEF. Going up against a Temptress with high WP? Switch out the support girl with the lust-based damage skill. And so on.
Hope that helps! You got this! :)