Training a parakeet to perch on my finger or shoulder?

I would like to buy a parakeet but im curious, will it allow me to hold it or does it take a lot of time for them to adjust to you and become comfortable

How do I retrain my first parakeet to sit on my finger after getting another parakeet?

I had a single parakeet for about 3 1/2 months. He was trained to sit on my finger and loved spending time with me. He would jump on his cage doors when he wanted to come out, which was quite often. I got another parakeet and now the first one won’t sit on my finger anymore. He constantly has to be next to the new parakeet and won’t let her out of his sight. I know this happens sometimes with two parakeets, but I miss having a trained parakeet. How can I retrain him?