If you are wondering about a single speed, just go to your multi-geared bike and try to find one gear that CAN work in all the situations you encounter. It will obviously be less than ideal in many -or most- situations. If, however, you can find a gear that works - that would be about the ratio you would want on your singe speed, though the wheelset can also make a difference. If it is a different diameter wheel that affects the gear ration, or if you are going from road racing tires to baloon tires you will want a bit lower ration to push the extra rolling resistance.

From there on, it is mostly up to what you prefer. There can be a wonderful simplicity to a single speed - clean bars - coaster brake - uncluttered chain lines - and there is beauty in this.

Since you have a single speed, just go ride it (unless you are trying to decide whether or not to exchange it) and see if you like it.

What type of single speed have you been gifted? Cruiser? Track bike? Vintage? or ???