Here comes another very easy achievement in this game.
I tackled it when I was around lvl. 5 (but you can try to do it earlier if you feel like it). Simply go to your first home kingdom and select "challenges". You will then find a list with several challenges to play. Choose the first one, selcet a few troops in addition to your hero and simply play through the fight.
You will get your first star after the first fight in this particular challenge is won.
BTW: The star system DOES NOT go together with your performance in the fight. You can fight as well or as poorly as you want. You just have to win the fight to get a star.
Hope this helps!
Any improvements? Please let me know in the comments below!