You're talking about telling the person that they are loved back, right ?
Because In English "I love you too" can unfortunately mean two very different things :
1. I love you also OR
2. I love you, but at the same time, I also love someone else.
わたしも好きだよ。 Is 90% fine for girls liking/loving someone in return. What's missing is any information on the situation you really need....age, sex, relationship, etc. that
could make this even more natural sounding.