To use the fill command go to the first block. Then type /fill then hit tab 3 times. Hit enter than go to the next block then hit tab 4 times. And then type which block you want to replace it with. In this case air and that will delete the blocks.
/fill x1 y1 z1 x2 y2 z2 minecraft:air
To use the /give command type /give your username for minecraft of @p for every player. Then type minecraft:stone which will give you stone. If you type wool 1 you will get white wool. Then if you have a number at the end of it like 27 you will get 27 of that thing.
/give @p minecraft:stone
To use a seed create a new world. Go to more world options. Then type this into the seed generator -702071384745498 make sure you have generated structures on. The hit done then create new world. You will spawn on a big island and right next to you you will see a big MUSHROOM ISLAND at spawn!