This is the self-introduction activity I use for Junior High School classes. I'll attach the file that I use for me, but you'll need to modify it for yourself, of course!
The gist of it is that before the students know much about me, it's fun to see what they assume. I do the activity in 5 or 6 groups, depending on class size. I ask the class if they know my name, and then ask them if they know how old I am or where I'm from. Since they usually don't know, I tell them "Let's guess!" and write the word "guess" on the board and explain what it means.
I ask students to get into groups and give each group one paper. I tell them their group number when I give them the paper, and ask them to write it. I encourage them to write any guess they have, no matter how silly or inaccurate it might be. While they're thinking, I write a huge chart up on the blackboard. On the vertical axis, I write each question (or a shorter summary of it like "How old?") and on the horizontal axis, I write the group numbers - "1 2 3 4 5 6 Jake". When one group finishes, I take their paper and write their answers up on their column on the board. This encourages other groups to finish, and gives them something to copy or modify if they can't think of a guess. If they have a funny guess, it's a source of amusement for the class. I return the paper to the group once I've finished writing.
Once all of the groups have finished and I've written everyone's guesses on the blackboard, it's time to check. I go through each question and each group's guess. "How old is Jake? Group 1 said 39. Group 2 said 100. Wow! Group 3 said..." Then I write up the actual answer in the "Jake" column. If any groups had the correct answer, I write "+2" in a different color of chalk, or "+1" if it was reasonably close (or if I found the answer funny).
After going through all of the questions, I total up each group's score. I run through all the groups, saying how many points each group got, with a pause for applause. Next I identify the champion group with one more round of applause. Now that the guessing game is finished, I run through my information one more time. "My name is Jake. I'm 36 years old. I'm from the USA. ..." After that, I ask the students if they have any questions. Depending on the class, you might get a lot, or you might not get any. If they seem shy, you can reward each student who asks a question by giving a point to their group. I'm game for any question, but if you don't want to answer something, you can tell them "It's a secret."
Unless you're extremely similar to me, you'll probably want to change the questions so they're more relevant to you. I'd suggest questions that relate to something unique or interesting about you - maybe you have a big family, or a food that you really like or dislike, or a hobby that the students might be surprised to hear about. I've added Japanese translations so that students can understand the question even if they don't understand the vocabulary or grammar. Don't forget to update or remove those! If you're not confident in writing in Japanese, you might be able to ask a JTE to help.