Since this is the very first class for 3rd grade elementary students, they have no experience with English whatsoever. The goal is simply to get them to greet each other with "hello!" as many times as possible, and to introduce themselves with "I'm ."
Use the PowerPoint to demonstrate the meaning. I use very common characters the students should all be familiar with. If possible, have the homeroom teacher act as the second character so you don't have to mime it all out by yourself. When you're finished, it's probably a good idea to go through again and have the students repeat and practice since they'll have to say one of the character's lines for the following activity.
After the introduction, show the students the instructions for the next activity. I've written them all out (with furigana) in the PowerPoint, so it should be extremely clear to the students, but in my experience, someone always finds a way to misunderstand something. Have the homeroom teacher help out with any students who aren't crystal clear with all the instructions. On slide 33, you'll need to replace my name with yours, unless your name is also David.
The gist of the activity is it's bingo, but active bingo instead of passive bingo. Give each of the students one card (see attachments, you'll need to make and laminate these beforehand) and one bingo worksheet. Since this is the first class, I've also included instructions to write their name, number, and class on the worksheet. Students will stand up, walk around the classroom, and find a partner. Then they will have the conversation on the card. "Hello! I'm Pikachu!" "Hello! I'm Mario!" Then they will find the appropriate spot on their worksheet and write the person's name who introduced themselves as that character. Then they say goodbye and find another partner. Once they get a bingo, they come to you to receive a sticker (if you don't have stickers, you'll need to replace these instructions with "stamp" or some other reward). They can get up to two stickers for the activity, but the second requires them to get a blackout. I've set the time limit as 15 minutes, which I believe should allow everyone to get one sticker, and some, but not all, to get two.
I've included a lesson plan written in English and Japanese, so your Japanese homeroom teachers can read it and know exactly what the plan is.
The font used in this is UDデジタル教科書. It should be on all of your school computers, but I don't have it on my personal computer at home. I prefer it because it has the handwritten lowercase "a" as well as other handwriting differences, but doesn't look like garbage like Comic Sans. If you don't have it, there might be some formatting discrepancies.
Credit to the respective copyright holders for use of the characters' pictures.