The Sims 3
50 Achievements
1,000
30-35h
You Never Forget Your First
Had a first kiss.
30
How to unlock the You Never Forget Your First achievement in The Sims 3 - Definitive Guide
To have your first kiss you must be in the love interest stage of a relationsip with someone. To get to this stage you will need to first be friends with someone and by choosing friendly options. Once you're friends that's when you should start choosing romantic options. Be flirty and do things like hugs and whatnot until the option of first kiss comes up. The option is literally called first kiss so if you got too far into the relationship without getting that option then pick some mean options to get the person back down to the right relationship level. Once you've kissed a girl/boy for the first time using that conversation option you will get the trophy.
After you manage to get a Sim to "Romantic Interest" status by using the romantic and friendly interaction a lot with that Sim. Click the romantic interaction on the Sim and select "First Kiss".
This is made a lot easier if you start the game with a male and female Sim.