When you write "Je suis entrée" you will be referring to a female subject, by the use of the past participle in the form shown.
The other point to keep in mind is that by using être as auxiliary you are using an "indirect" form of the verb which will need to be followed by a preposition (this is intransitive usage).
If you were to write "J'ai entré" the subject could be male or female and the verb would need to be followed by a noun. Why? Because this would be transitive usage and needs a direct object after the verb to complete the meaning.
So the point for you to understand is this matter of direct or indirect usage -- with a direct object (avoir) or with an indirect object (être) and this is technically referred to as transitive or intransitive usage.
Hope this aids your understanding.