The golden rule of Minecraft is: Never dig straight down or up.
It is possible to stand on a block while destroying it. Once it's destroyed you will fall down one block to whatever is beneath it. The problem is, there is a significant chance you will either stumble upon a cavern and take fall damage or more unfortunately, fall into lava. If you do land in lava move to water immediately, if any is available, to save your life. (Even if you can't save your life, try to get away from the lava before you die, so it won't destroy your possessions.) If you survive falling into a cavern, you will then have to mine your way back to the surface. If you dig straight up, then you can dig out a block holding back lava or sand/gravel and also die. Even water could kill you if you aren't able to get away quick enough
Note - if something is taking a long time to break, stop hitting it unless you just want to remove it; it will disappear when you finally break it - return to it after you have the right tool to retrieve the item. 3 important rules to note are:
- The better the material a tool is made of, the faster the tool retrieves and breaks items
- Wood-related items requires no tools to break and obtain the resource, but axes may be used to speed up the process.
- Coal and stone can be mined with a wooden pickaxe, but iron and lapis lazuli require a stone pickaxe, and all other ores require iron tools or better. Obsidian, however can only be mined by a diamond pickaxe.
At this point, if you have some daylight left, you should probably start working on your shelter. Once that's done, you can kill more animals, collect some things or just explore. If you don't feel like it, just skip to Night time.