Hi June!!! I don't have very many tips since I recently did my first babysitting job myself(to be fair I had Tyberious with me so they took care of the 2-year-old and I was with the 5 year old for two hours) but a helpful tip I have is to ask the parent questions like;

Are they allergic to anything?

Is there anything they are not allowed to do?

Is there anything I should be aware of before babysitting them?

Do they have anything specific they need to do while I'm with them?

How long will I be with them?

Who should I call in case something goes wrong?

And other questions of the like. 

You'll be fine! Babysitting is fun and generally fairly easy. It's pretty much just playing with the kids and keeping them happy. They'll be excited to have you come over because they see you as a cool, older kid who's coming to hang out with them. Just make sure you act as the one in charge. You're allowed to say no if you have reason to. This is your first time coming over, so they'll be gauging how much they can get away with when you're here. Be assertive. 

If they're old enough you could bring a board game to play with them! Kids love new things and it will keep them occupied for a while. Just make sure all the pieces are there and you know the rules before you go over. If you have any interesting skills you could teach them, like sewing or drawing, bring some supplies. It will give you more options and make them look forward to you coming over.

Most importantly, have fun! The kids will probably already have planned one million things to do with you, so let it happen. If they aren't interested in the things you brought, that's ok. As you babysit them more you'll get to know them and learn what they're interested in. If you're unsure about something, text the parent. You'll do a great job, June!