This is single speed rather than fixed? You need a freewheel removal tool for the right sort of freewheel. Take the axle nut off, put the tool in place, put the axle nut back on loosely but tight enough to stop the tool slipping, and then turn the tool with a big spanner. Easy. The tool is cheap and you will always find a use for it.

If it's fixed rather than single speed, you need a C spanner to remove the lock ring, and a chain whip to loosen the sprocket. Again, cheap and readily available tools that you will use time and again.

As the sprocket is smaller than the chainring, 1 tooth difference on the sprocket makes a bigger percentage change than 1 tooth difference on the chain ring.

For those who like to skid on fixed, there is all sorts of spooky magic about this number divided by that number to get the most skid patches to even out the tyre wear. If you ride a freewheel (or fixed with brakes) that stuff is irrelevant.