The Sims 3
50 Achievements
1,000
30-35h
BFF!
Made a best friend.
10
How to unlock the BFF! achievement in The Sims 3 - Definitive Guide
If you want, you can make one sim with the trait above and another with the "Snob" trait I believe it is. Snobs love being complimented, continue praising them, complimenting them, ect, vanity....
To talk to someone hover over them and push . This will bring up their menu of talking options. If you're just meeting the person for the first time then be nice to them and use your traits to your advantage. For example if you picked the trait of being funny then you get a bonus on telling jokes so tell jokes more often. You can pick out your trait's special conversation abilities because of the picture next to the conversation option.
Now that you know that just keep talking to the same person for a while. You're going to want to keep talking or playing games or doing whatever else they like to do. You can see how much they like you by the bar that's displayed in the friends menu. Once they reach about 90-100% of the bar you will have reached best friend status. The game will come up with a screen saying that you are now best friends and soon after the trophy will unlock.
To make a best friend all you have to do is keep being friendly to a Sim until they become your best friend. This is best done on your boss or co-workers because you will need to become best friends with them anyway for the Working Friends achievement. If you have started off with more than one Sim you can simply befriend one of them because you won't have to go out of your way to find them. Refer to Working Friends for more info.