I guess I could go into things that I consider prior to starting my first war.
1) National borders:
If I share 6 provinces with one nation, and 1 province with two other nations each, then that 6 province neighbour is both significantly more open to my attacks, as well as able to make a much wider attack at me. By contrast those 1-border neighbours will be easier to defend via that one chokepoint. Sometimes there are borders that are mostly river or mountain crossings too, which would make supporting a prolonged offensive much more difficult as well. If all other things are equal (they never are) I go after the high risk guy with whom I share the most border.
2) National rock-paper-scissors:
Different nations have different strengths, weaknesses and power curves (depends on various nebulous factors of course, but it's fairly safe to say that for ex. C'tis is a strong midgame nation with potent skelespam, but is a bit weak before that and probably won't be relatively as powerful later on). If I feel that I am relatively more powerful than my opponent at the moment, I attack. If not, I'll bide my time.
3) Power projection:
If my nation is 8 provinces wide, and most of my map-move 1 expanders are on the western edge, it's going to take 7 turns before I can bring the majority of my forces to bear. I'd rather not "waste" 7 turns like that. In that case I'd much rather attack whoever is on my western border, simply because I can start conquering (and thus getting more gems/gold from new provinces) faster.
And I suppose I could put "4) Player personality" here as well, though it doesn't factor so much into this game as I haven't played with most folks here. I generally try to avoid carrying grudges, but it is true that if someone time and time again blatantly lies to you you will remember it. Likewise someone who seems preternaturally loyal is probably someone who'll you trust a border more easily. But at the end of the day I look at the three factors above and go with those. There are no friends in this game, only temporary allies of convenience.
And a special mention to our newbie C'tis player. I'm not sure if you're familiar, but C'tissians are cold blooded. What this means is that they suffer +1 encumbrance in neutral temperature, and +3 for every cold scale thereafter. So attacking anyone with cold scales is quite dangerous as your troops and mages will pass out a lot faster. That's something you need to consider. For consolation I can mention that my first war in 408 was against a cold-3 opponent but I managed ok since I brought in a lot of mages and spammed skeletons (mages passed out, but I still got a ton of skeles out and skeletons don't mind the cold).