This happened with me just a few days ago! I had a comment by someone I didn't know, checked their account and their comment to me was their first one, (and they chose me as their first person to follow - what an honor!) so I wrote this comment to them. (I wish I'd seen all the great advice here first!)

I see that you have just joined Steemit - welcome! I don't know what your interests are but there will be people here who share them. And you will find very nice people here who will also welcome you gladly!

I was given good advice when I first joined, to make thoughtful comments (like you did with me) as well as writing your own posts. And to make a good introductory post. Use your own photos if possible, and if you use photos from the Internet to make sure they are copyright free (Pixabay.com, Pexels.com) and give credit to the sites they come from. If you like to enter contests there are many fun ones here and people will get to know you through them. I hope you enjoy your Steemit journey!