It's easy to convert a bike to a single speed.
Primarily, you have to remove everything that used to be part of the drive train (crank arms, front and rear derailleurs, shifters, cables, front chainrings and rear sprockets).
The rear hub & cassette will need to be replaced as well.
Once you have everything off, you just replace it with the right rings (ie. 45/16) and chain. The only "hard" part is making sure everything is aligned properly.