My car is sluggish until I've driven it about a mile or so then it runs fine. What could be wrong with it.?

Maybe it's a vacuum leak. Is the check engine light on?

Don't guess this. Get the OBD2 trouble codes pulled, then work from there to diagnose why parts or systems are malfunctioning.

Lots of different possibilities but with the lack of detail you've provided with your question, unfortunately no one will be able to narrow down that list...

Only when the engine is cold, or even when it is warm but it's been parked for a short time?

Details, Richard, we need more details.

Get a "tune up" and see if the issue goes away.

I had that problem on an old car once and could not figure it out. Until I was stranded at walmart. I went back the next day and got it and took it to my mechanic who said it was the fuel filter or the fuel pump. They replaced the filter first because it was cheaper but said that did not solve it. (I can't recall how they knew that but they did, so they put a new fuel pump on it and it was fine after that)

Stuck thermostat. Choke not working. Inexperienced operator. Lots of things could be wrong. Take it to a mechanic.

Make, model and year please.