Converting front crankset to a single type on a 18" Carrera Vulcan MTB bike
If you now have a triple front chainring, the best position for new a single chainring replacement would be the middle position on your current crank. It will likely to give you the best chainline for the extreme rear cassette positions (the smallest and the biggest rear cogs).

Regarding the number of teeth, it is up to you to decide, but use your existing middle ring tooth count as a starting point. E.g., if your current front rings are 42/32/24, the middle ring has 32 teeth. The new chainring should have 32 teeth or less if you often run steep uphill, or 34−36 if you focus on fast downhill. Note that if you do not feel comfortable using only your current middle front chainring (e.g. only using rear derailleur to change gears) already now, it means that the range of your present rear cassette would not be wide enough for you when you switch to a single front chainring. In this case, consider getting a wider range cassette as well. Be sure to match BCD on new and old stuff.

If you plan to get a new chainring which is marketed to prevent chain drops (often called "narrow/wide" chainrings), and/or if you have a rear derailleur with clutch, you may be fine without a chain guard (still recommended for riding rough terrain though). If you plan to get an old-school (cheaper) plain chainring meant to be used in single-speed drivetrains or one with ramps for front derailleur, you'll definitely need a chain guard.