The first thing you need to do on your first day is get around 20-64 pieces of food and a bed and if you find a village it makes it a lot easier
Next you need to go get iron there are four ways to get iron.
1. Explore caves
2.Strip mine at Y level 16
3.Go exploring to find a mountain and get the iron on the peak
4.Make Ianxofour's iron farm.
Neext you need to get some daimonds.
1. You can go and find deepslate caves and serch arownd the cave for diamonds.
2. You can go trip mineing at Y level -58 you should also bring a trappdoor to crawl at Y level -58.