To learn fixed gear, better start with high gear and then get lower, or the opposite?
In general I would recommend starting low. Your legs will go through an adjustment period and you may find that your knees get sore from a larger gear. After you've ridden for a few weeks pay attention to how you're riding. If you're in a hilly area are you struggling to get up the hills? Do you find that your cadence is regularly high enough that your hips are bouncing on the saddle? There are both indications that you should adjust your gearing high or low respectively. 

It's can be cheaper to replace a track cog than a front chain ring so start with 16T in the back and then pick a chainring that gets you into the gear you're looking for. With any luck, if you do need to make an adjustment up or down switching the rear cog won't force you to change your chain length.