There is a positive correlation between rating and player's attributes.
You can take the following ways to increase player's attributes:
1. Recruit high quality players: high quality players (S+) have higher attributes. You can spend Gems to recruit them in Shop, or use Euro to buy them in Transfer Market.
2. Activate Chemistry: try hard to activate the Chemistry of your main players which will greatly increase their attributes.
3. Upgrade Skills: you can upgrade players' skills in Training Base to improve their attributes.
4. Unlock: players are able to unlock the skills when they complete specified drills.
5. Evolve: after unlocking 4 skills, a player can Evolve, and Evolve will greatly increase your Rating.
6. Enhance: Enhance can transfer player's potential into real attributes.