My first was alto saxaphone. There's an octave key, so it's easier to go up in octave than for instruments like the flute or brass instruments that require much more breath control to do that. I found alto sax to be a good "gateway" instrument. From there I went on to tenor and soprano sax, bassoon, flute, trumpet, and later, percussion, etc.
A lot of people start off with guitar. My kids are starting with piano. Violin is also a popular first choice.
Think about what kind of music brings you the most joy and choose a compatible instrument. If your favorite genre is Spanish acoustic guitar, go for guitar. I chose sax because I love jazz. My son picked piano because he loves my husband's Thelonius Monk albums and feels drawn to the mathematical and technical aspects of it. My daughter chose it because it's such a natural support for her voice lessons.
Just pick what you think you will enjoy most. That love for it will make it easy.