Instructions
The Natural Way
Start the game, left-click on the Sim that you want your character to have a crush on and select an action from the menu to interact with her.
Click the "Relationship" button --- the icon of two characters holding hands on the bottom of the screen. You'll find it just below the "Needs" icon. It shows you the relationship between your Sim and other Sims. The top bar on the chart is the daily meter, and the bottom bar is the lifetime meter.
Keep interacting with the other Sim until your character's daily meter is above fifty points. This will turn the other Sim into a friend. You'll know your interactions are successful by the small green plus points appearing above the Sims as they interact.
Continue interacting with the other Sim and select romantic interactions such as "Charm" and "Flirt" from the menu until the relationship meter is above seventy points. You'll see a small pink heart appear below the other Sim's portrait in the Relationship menu, indicating that your character has a crush on her.
Cheating
Start the game and wait until you are in control of your Sim. Press "CTR," "Shift" and "C"; an input prompt will appear at the top of the game screen.
Type "boolprop testingcheatsenabled true" into the input prompt and press "Enter" to activate the cheat mode.
Interact with the Sim you want your character to have a crush on, and click the "Relationship" button to see the daily and lifetime meters.
Left-click the daily meter and then, while holding down the left mouse button, drag it to the right until it is at seventy points. Do the same for the lifetime meter so that it is just below sixty points.
Interact with the Sim who you changed the meters for until the pink crush heart appears underneath the Sim's portrait in the Relationship menu.