"Kiss a Sim" appears as a Daily Wish sometimes. If you have your Lover, you can simply visit him/her to perfrom the action. Unless you need to get a romantic interest and get the relationship level up to 3 to unlock the relationship event to change the status into Lover.
Kiss a Sim
Kiss a Sim with whom I am romantic
1. Perform Firty introduction
At first, you need to find a Sim and perform Firty Introduction to him/her. Then your romatilc relationship with the Sim starts.
2. Get relationship level up
Once you find a Sim you like, let's get relationship level up by performing flirty actions inclusing Hold hands and Ask about day. When the level reaches 2, the relationship becomes Romantic interest. When the level reaches 3, the relationship is still Romantic interest, but "Kiss" appears on the list of actions.
3. Perform the action
Once you get a Romantic interest Sim with relation ship level 3, talk to him/her to perform "Kiss" action. Then the Daily Wish is achieved.