There is a girl who wants to become Muslim, but she is living with a Christian family who eat pork. Can she eat with them if she becomes Muslim in secret? How she become Muslim and pray and wear a hijab?
She can become a Muslim by taking the Shahadatain:
"ashadu an la ilaha il Allah, wa ashadu anna Muhammad dur Rasul Allah"
"I testify that there is no god except Allah, and I testify that Muhammad is Allah's Messenger "
The answer to your remaining question depends a lot on factors like her location, age, means, the attitude of her family and society etc.
She should declare and practise Islam openly, unless she knows that doing this will endager her life or open her to persecution.
If she is unable to practise because of persecution, then she should migrate. She can do this if she has the means to migrate, such as finances, a plan of escape etc. Islam even allows a woman to migrate without a mahram if the way is safe.
Indeed, those whom the angels take [in death] while wronging themselves - [the angels] will say, "In what [condition] were you?" They will say, "We were oppressed in the land." The angels will say, "Was not the earth of Allah spacious [enough] for you to emigrate therein?" For those, their refuge is Hell - and evil it is as a destination. (an-nisa 97)
And whoever emigrates for the cause of Allah will find on the earth many [alternative] locations and abundance. (an-nisa 100)
If she can not currently migrate, then she should wait and work to acquire the means to migrate. Till that time she can hide her faith and practice as much as she is safely able to. For example she may be able to pray while locked in her room, and she may present an excuse for not eating pork (she can say she doesn't like it, or that it makes her stomach sick).
Except for the oppressed among men, women and children who cannot devise a plan nor are they directed to a way - For those it is expected that Allah will pardon them, and Allah is ever Pardoning and Forgiving. (an-nisa 98)