I wrote a story a while back ago on a different site and in it, I was trying to write these two characters who absolutely hated each other having an argument.
That was it, they weren't in a relationship with each other, they weren't friends in the past or anything, they just genuinely hated...
If you're worried you'll lose motivation after the beginning, or that you won't know what to write, I'd say before you even start you should figure out the ending, then move back from there. Knowing where you want to go is always good, and it's helped me at least.