It’s a great occasion of your life, so wear something nice. Test it beforehand to make sure it’s fitting nicely and there’s nothing to cause you problems (like a slippy neckline or too short a skirt/pantleg). Make sure you have an alternate outfit in case you get nervous and spill something, extra hose if you’re wearing that, comb, etc. (Obviously this is pretty standard advice for any big occasion.)
Basically, don’t worry too much about it, but maybe do more preparation before the night than you usually would. Your brain may be fried on the actual night.
Dress your heart up, too. Pray. Ask God for blessings for you and your spouse and your sponsor, and all the sponsors and confirmandi, and the priest, and everybody else. Ask your friends to pray for you. Disposing yourself to receive all those new graces you’ll be getting is a very good idea!
Re: oil – Obviously black is a good color for accidental holy oil spills…
It’s perfectly okay to have something ready to blot up chrism. A lot of RCIA programs provide stuff for you for this, and then take charge of it afterward. In that case, you won’t have to worry about it. But in case yours doesn’t, I’d have a cloth ready. (And probably a plastic baggie for the cloth, too.)
Long ago, what was used was a cloth with a sort of woolly pad in it (often tied around people’s heads temporarily, like some kind of medieval headband!), but something like an absorbent cloth is okay, or even tissues. The chrism is a sacramental, though, so either keep the now-oily cloth around the house as a holy sacramental (the medieval thing to do!), or burn it, or bury it. Or give it to the RCIA director to do that stuff.
Don’t put an oily cloth anywhere near an open flame, unless you really mean to set it on fire! Your guardian angel likes having you somewhat safe.
, but I wouldn’t wear my favourite new blouse or expensive new dress either, if there were another option.