You can do anything you want - it is your bike.
- To test singlespeed (and help figure out what gear ratio you want) pick a gear choice, and do your normal rides without changing. Doesn't matter if you have a hill to climb or a stiff headwind, stay in the same gear. That's all you'll be able to do in the future.
And its okay to just stay there if it suits you. So far, this is a no-cost change.
This #2 section presumes a cassette, whereas your
bike has a freewheel. Thus the middle of this
answer does not work for OP's bike, but leaving
- Once you've chosen a gear, count the number of teeth on both cogs. You can buy a cassette replacement that only has one cog, and looks something like this:
Example shows a 13 tooth, but seems to be available in 13/14/15/16/17/18/20 tooth options. There are shims and spacers to fill the spare, and you can put them on either side of the single cog to get a straight chainline.
On the front, you probably have three chainrings. It may be possible to unbolt the chainrings you're not using, or the crankset may be rivetted together. It is possible to drill out rivets, but those cranksets may need the added strength of a steel sandwich, allowing one single chainring to bend if left unsupported. In the first instance, you could just leave them in place.
You can choose to remove the front derailleur, cable, and shifter. Sometimes the front derailleur can be locked-out using the limit screws so its only in one position, to act as a chain guide and prevent the chain jumping off.
2b) Your other option is to replace the entire rear wheel with another using a cassette-based interface, or one designed for a BMX style of single cog.
- Lastly, you need to fit your new chain. The chainline must be straight, meaning the cog at the rear is in-line with your front chainring.
Your bike probably lacks any way to adjust the chain's tension using the wheel placement in slots, so you'll need to either fit a tensioner like this one:
Or it may be possible to hack up your existing rear derailleur to provide tension on the chain, and use the limit screws to hold it in place. Remember you want as many of the teeth engaged in the rear cog to the chain as possible; ideally half of the teeth or more, and not 3/8 like in a derailleur bike.
Additionally, you still require two working brakes. Single speed offers zero ways to slow down.
I suggest you store all the takeoff parts, in case you want to revert later. Single speed is not all it is cracked up to be in terms of rideability, and offers simplicity with a loss of functionality too.