Check out this guide posted on Carl's Sims 4 Guide website. A lot of great information on how to make friends in text form and a YT video if you'd like to take a look.
The Sims 4 Relationships GuideOne of the best ways to make friends that I personally rely on is the cloud/stargaze interaction available if you have the Outdoor Retreat GP. Increasing their Charisma skill is also a good idea. As they level up, special social interactions are unlocked that are a big boost to making friends.
For romancing Sims in addition to the romantic aura lot trait already mentioned, have one of your Sims paint a flirty painting. Enable the romantic aura and hang it in the room where the two Sims are chatting. Or if you have those TS3 emotion lamps, could enable the flirty one in the room as well.
Also, when they are in a flirty mood, have them bake flirty cookies. Any Sim that eats one will become instantly flirty. To get your Sim flirty have them watch the Romance channel on TV or 'Freshen up' using a mirror.
If you have the City Living EP, you're golden for romance. Just send the Sims you want to get together to a Romance festival and have them drink the tea together. Bingo, instantly flirty and receptive to romance. :) I always go into build mode during a Romance Festival and add a closet to the Karaoke bar somewhere too. Just in case they want to 'seal the deal' on their new relationship. As a bonus, the festival will shoot off fireworks for them when they woo hoo. ;)
One important tip about romance, DO NOT ever use the 'Confess Attraction' social at the beginning of a romance. That social is guaranteed to be a giant buzz kill.
Hope this is helpful for you. Have fun! :)
