Formal:
* Je l'aime bien. (I like her well.) This is a neutral way to say you like someone.
* Je la trouve sympathique. (I find her likeable.) This implies a positive opinion of her personality.
Informal:
* Elle me plaît. (She pleases me.) This implies attraction or romantic interest.
* Je l'aime. (I like her.) This can mean "I like her" in a general sense, or it can be more romantic depending on the context.
* Elle est cool. (She's cool.) This is a very casual way to say you like someone.
More specific:
* J'aime son sourire. (I like her smile.) This is a more specific way to say you like something about her.
* Je l'admire. (I admire her.) This suggests you have a lot of respect for her.
The best way to say it depends on the context and your relationship with the person.