Step 1: Use lewis structure guidelines to draw the lewis structure of BF3.
Step2: Apply VSEPR notation, A X E
A=Number of central atoms
X=Number of surrounding atoms
E= Number of lone pairs on central atom
For the above molecule VSEPR notation will be AX3E0
Step 3: Use VSEPR table to find the shape.
AX3 has trigonal planar shape. So the shape of BF3 molecule is trigonal planar.
Download a copy of VSEPR shapes table here