If you're the soloist, you lead. The piano accompanist follows. But you can help them out a lot (a lot!) by cuing them in for things. Take a breath and lift the instrument (?) and drop it down on the beat where you come in, like a conductor. Breath/rise, play.
Decide on tuning ahead of time. Make sure the accompanist knows what note(s) you want to tune to. If you can, just tune before the performance and then it's just a quick double check on stage. It might be a detail at this point, but tune all your notes to the piano, not to the perfection that strings can have.
If you play in a string ensemble, you're probably used to playing with an ensemble's time. If not, get used to that. Play along with the metronome. If you mess up, jump back in. You can't go back and make corrections with an ensemble. Part of your attention needs to be on them too.
I would just practice your part well and be able to play it with good time. If it's new, try to meet with the pianist more times. I'm thinking maybe three, but that's just me. Otherwise once or twice is fine.
Take some notes of stuff to improve on in the future too. That makes the whole thing a guaranteed success -- You always walk away knowing things to work on for the next time.
When you stand, make sure you're positioned so the pianist can see you. You face the audience. The piano is usually angled in some way so the accompanist can see you. That's why the visual nod and even hearing the breath can help. I've heard some string players really breath loudly, so I doubt you can overdo it.
Topic: Playing with a piano...Help