How do I get the Fleeca Heist mission in GTA V?
From your question body, it sounds like you haven't even begun the heist tutorial. There are several requirements that must be met before Lester will contact you.
From the GTA Wiki
Players must be rank 12 or above to set up a heist, and must own a high-end apartment with a heist planning room. Once the player has reached the necessary level and purchased an appropriate apartment, Lester Crest will contact the player by text and follow up phone call introducing the heists concept. The player must then meet with Lester at his garment factory. This will launch the tutorial heist The Fleeca Job.