Scrap is a very important resource in Rust and is used to research blueprints and build components. By the mid-game, the need for scrap will be pretty high, and by the late game, it will be essential.
Scrap be found in many different places. Barrels, crates, and boxes can all give scrap, and military crates normally give a little more scrap that those sources. Elite military crates can spawn at monuments, giving up to two dozen Scrap, but these can be hotly contested, dangerous places to be, especially if you are alone.
Junk piles that are found on The Road can also be searched, and will yield plenty of Scrap, but can once again be a bit dangerous if you are on your own. Monuments like the Mining Outpost, the Abandoned Cabins, or the Supermarket will all contain Scrap as well. Pretty much all monuments except the Dome will have a recycler, so are good places to farm scrap if you are feeling brave. Even though the Dome doesn’t have a recycler, it can yield a huge amount of scrap which you can then recycle later.
To get a lot of Scrap quickly, and reasonably safely as a new player, you want to make your way to an outpost. Outposts will not only have scrap, but they will also have a recycler. Gather up as many components and scrap as you can, then pop them in the recycler to generate much more scrap.
If you manage to find a scuba tank, you can make for the shore and use a boat to explore the sea. There will be various lumps of floating debris marking sunken ships, and you can use the scuba tank to swim down and explore them. If you find large sunken crates, these can contain a lot of Scrap.
The real trick to building up a lot of Scrap quickly is to not lose it to other players before you can get back to your base, so make sure you play carefully and stay alert for threats.