Here's a comprehensive list of what you should do with your freshly-harvested diamondd depending on the amount you have:
5: pickaxe + sword. Alternatively, make an enchanting table
6: pickaxe + axe / pickaxe + sword + shovel depending on area (choose shovel if you're gonna dig, axe if you're gonna chop some trees down. Axe and sword both serve as weapons)
7: pickaxe + axe + shovel
9: pickaxe + axe + shovel + sword
1 476: fully powered diamond beacon
30 564: ten thousand one hundred and eighty-eight AXE so you don't smell as bad
2 965 493: coat a small island with a layer of diamond blocks
4 000 000: two million hoes (you still won't have as many as me, but it's worth trying)
4,6 × 10^18: fill the entire playable world with diamond blocks
5,7 × 10^5848: buy the actual Mojang company
There you go! Hope that helps or keeps you busy on this fine evening!