to warm up, just play some slow scales or slur exercises, or maybe do a right hand exercise if it your right hand that is sluggish when you start off.
that piece you are playing, you could take out any single measure and use that as a right hand warm up exercise, too.
the best warm up is any exercise that you play slowly and meditatively so that the blood can get moving in your hand. So really anything that fits that description can be a useful warmup.
as for your metronome, in 6/8 I will either put the click on beat 1 and 4 (2 clicks per bar) or on every beat. If your part has a lot of 16th and 32nd notes, putting the click on every beat while you are working everything out can help you place everything while you are working up the piece. 6/8 though should feel like a duple meter when you perform it.
so if you put a click on each beat, then you will play 2 of those 16th notes on each click
if you put it on 1 and 4 and have 2 clicks per bar, then you are playing 6 of the 16th notes per click. The 16th notes are probably printed so that they are grouped in two groups of 6 in each bar.