I'm assuming you're talking about building a friend's self-esteem here, so a few ideas:

1. Give them compliments on things that are within their control! Compliment their style, their taste in music, their laugh, things that aren't just superficial or genetic.
2. Remind them of how great you think they are. Even if it's a random text saying you saw something that reminded you of them, let them know that you're thinking about them and that they're an important person in your life.
3. Go out with them in no-pressure social settings. Sign up for one of those paint-and-sip classes and chat with the people next to you, hang out at the dog park if you have a dog and talk to other dog owners. Build in casual small talk with strangers, it's good practice.
4. If they struggle with anxiety or mental health issues, let them know that you are there, you're not going anywhere, and that if they want to talk, they have you.