Well, Japan is one of the strongest (if not the strongest) for a tech victory due to the very early +1 food on all sea tiles.
For domination, I found the Aztec to be a perfect civ to win a domination victory with as well as the perfect civ to get used to the game with. The unforgiving nature of combat in this is almost negated by the Aztecs.
But seriously, Japan is great for a tech victory. What I always do when I play them, regardless of what victory I am going for, is to build a galley as soon as I have 3 warriors out and then build a settler. After that, I put my settler on a galley and move him to an island - any island really but the smaller the amount of land the better. You don't need hammers here. This town will be your early tech and guaranteed quick expansion.
Sometime in this time I'll have 100 gold and will have a free settler. I usually put him in a spot with high hammers - that will be my production town for the majority of the game. I use the 100 gold to connect it with my main city so I can move archers there.
My main city is used for building lots of coastal or island cities. That's all I use it for when playing as Japan (the reason is because I stick with Republic for a long long while). Your goal as Japan should be to own as many Islands as possible. This ensures you either a pure economic advantage if you devote to gold or a pure tech advantage if you devote to tech. Either of these can be used to give you a numbers advantage via gold or a strength advantage via tech. Both are viable.