FWIW. As
dabac has indicated, a "fixed gear" is probably not such a good idea ... at least, IMO, too ... but, I am presuming that you are riding on roadways vs. a bike path ...
IF you find that you NEVER shift because you ride in a relatively FLAT area with minimal elevation change (
vs. not knowing when to shift), then a "Single Speed" which is capable of Freewheeling (
"coasting") may indeed be a better option ... at least, initially ...
For an efficient Single Speed, you will probably want to determine the most efficient gearing for YOU (
or, the training which you may be doing at the moment) rather than simply opt for the common 42x16 gearing which you will find on most ready-to-ride Single Speed bikes OR which will possibly be recommended to you ...
- try several different chainring/cog combinations on your current bike where you ride ...
- ultimately, with a Single Speed, the gearing you choose will be a compromise
BTW. Let me reenforce what
dabac mentioned that going down hills where the coasting speed might exceed 20mph can be troublesome on a "fixed gear" bike.