When I am acquainted with religious knowledge, do I become a mufti on simple matters?

Issuing fatawa is an honorable and high ranking position, since the mufti fulfills some of the responsibilities of the Prophet in the community and acts on his behalf in delivering rulings. Imam Al-Nawawi said in al-Magmu‘:

Take note that formulating legal opinions is the greatest of dangers, a grand position, and is full of merits. This is because a mufti is heir to the prophets, peace and blessings of Allah be upon them, and fulfills the communal obligation. However, he is subject to errors. Because of this, it is said that a mufti is the legal representative of Allah. (1:40)

There is a huge difference between knowing one or several legal rulings by being well read on Islam and reporting it to others – this reporting to others is not called issuing a fatwa – and being a mufti who conveys the teachings of religion from Allah and instructs the masses on His intent, and knowing how the ruling of Allah applies appropriately to a situation in a manner which accomplishes the overall objectives of the Shari‘ah and which is consistent with the general interests of the people.

Allah the Almighty knows best.