metablue wrote:No wonder it sounded a bit weird! I mustn't have heard it before. You would just leave it off altogether, or is there another desu variant?
You don't use anything at all.
Plus, as I recall (sorry if I'm wrong), you're a girl/woman/lady/female (choose whichever you find appropriate), right? Women usually don't use "da" at the end of their sentences. It's not particularly femine. Some younger women do it, but they generally grow out of this habit.
As a general rule:
Women use "desu" to be polite, otherwise they use nothing.
Men use "desu" to be polite, otherwise, they use "da" for -na adjectives, and nothing for -i adjectives.
As Jason said, "da" doesn't follow -i adjectives. I once said "tsugou ga warui da" before I knew this rule, and I was told that this mistake sounds very childish, like baby Japanese.