Attention, people with ADHD, autism, and anyone who has trouble with talking to people and keeping friends:
Hello! I'm one of you. When I was younger, I would have so much trouble having friends. I'm still not very good at it, but I will tell you what has made it better.
1. I learned the local sign language alphabet. When people are talking, my head fills up with things I want to say. I used to either interrupt people, or I would be so focused on not interrupting and waiting for my turn that I wouldn't actually listen to them.
Now, I use the sign alphabet to remember what I want to say. So if I want to talk about dolphins, I hold the D shape. I leave the remembering to my hand. This lets me listen to what the other person is saying. You could use a notepad or a phone, as long as you do it quickly.
2. I learned how to engage the other person. This is so hard. I want to ramble on and on! But people generally don't like that. They want to talk with you, not be talked at.
I would make pauses in my speech, so they could talk. I would ask questions. I wasn't very good at asking questions at first. I would say, "isn't that cool?" That doesn't give someone much to respond to. So...
3. I learned to ask open ended questions, like "What do you think about this character's choice?"
4. I learned that I needed to find common interests. I'm lucky because I can talk about a wide variety of topics. If you can't, I suggest you find clubs or forums for your special interest. You could also try to find something related to it. For example, if you like Steven Universe, and the other person is interested in gem stones, you could ask them about the real life properties of diamonds versus pearls and things.
As much as I want to talk all day about the show I just watched, I need to find someone who wants to hear it. If I'm with someone who doesn't like that kind of thing, I need to talk about something we both like. If I just met them, I can ask what their hobbies or favorite shows are, or I can ask them if they like my favorite stuff.
Sometimes I can get my friends into the stuff I'm into, but that's not always the case. And that's okay. I can open myself up to trying out things they're interested in.