There are several possibilities, for which we also have canonical sentences, {bI'reS} and also {wa'DIch} used as time stamps:
{bI'reS} 'beginning, prologue, first, initially, after' (used also as a time stamp at the beginning of the sentence)
1) bI'reS qeylIS vaq molor.
«First, Molor taunts Kahless.»
2) may' bI'reS bejtaHvIS mon.
«First, he watches the battle smiling.»
3) tera' DIS wa' Hut vagh Hut, bI'reS puv 'amerI'qa' 'ev chan 'ev X-wa'maH vagh. muDDaq neH muD Dujmey vorgh lu'orlu', 'ach loghDaq puvlaH Dujmey 'orbogh nuv 'e' 'agh X-wa'maH vagh.
«The North American X-15, initially flown in 1959, bridged the gap between piloted flight in the atmosphere and piloted space flight.»
{wa'DIch} 'first, original, at first'
4) wa'DIch nach 'ay'...
«First Nach, the form of the head, ...»
5) wa'DIch Hegh moratlh.
«First, Morath fell.»
So it seems, both are fine. I also do not see a semantic difference between them.
Also consider {-pa'} 'before', which you could use for English "first", e.g. "If we want to open this door, we first need a key." --> "Before we open this door, we require its key."
Greetings from Zurich,
- André