Potřebuji SIM kartu.
When to use
This is a direct statement to use when you are at a mobile carrier's store, an electronics shop, or an airport service counter and need to purchase a SIM card.
How it works
The phrase means 'I need a SIM card.' 'Potřebuji' - I need 'SIM kartu' - SIM card (accusative case).
Grammar
The verb 'potřebovat' (to need) requires its object to be in the accusative case. 'Karta' (card) is feminine, so in the accusative, it becomes 'kartu'.
Notes
State your need for a SIM card at a mobile shop or airport.