Best way to say: Sure/I will

[marrow]

I looked up しょうちんのすけ but I've never used it first hand and I'm not sure how old fashioned/correct it would be when talking with a stranger my own age.

Since I'd like to tell this person I will comment I was thinking of just saying "write" in but I'm sure there's a better way.

I suppose I could just say はい or ようかい?

[jazzbeans]

Not sure about the standard and not 100% on what the context is but, I personally say "はい、もちろん！". Yes, of course!/Yeah, sure!

Haven't noticed that it's inappropriate.

[Javizy]

I suppose I could just say はい or ようかい?

Do you mean 了解（りょうかい）? That's okay if you're speaking to your commanding officer when he's just given you orders. You can use it as a joke, but it's a stiff way of saying わかった.

If you mean 'sure' when somebody invites you to do something, you can say one of these, among others.

はい (stiff, and unusual with friends)
ええ
うん (informal)
いい（です）よ (I think this is closest in nuance to an enthusiastic 'sure!')

[marrow]

Yeah I meant りょうかい--I was just typing too quickly apparently. ^^; Back in Japanese class (when I was in high school) we used to use it all the time jokingly but I wasn't sure how it might come off in an informal friendly conversation.

It's always tough on the internet especially, because you don't quite type how you speak nor how you write.

Thanks for your advice guys. =)