fish oil is always a good one , I usually buy whatever one is on sale they are gel caps and I give 1 cap to my smaller dogs every day and 2 to my larger. you can get the liquid kind if prefered it just has a pump and pour it over there food.
I also feed garlic with brewers yeast I find it helps control fees naturally without the advantage or other toxic things on the market, Its 1 pill for every 10lbs every day. Works great for us.
If your dog has any specific issues joint problems or what not you can add a couple other things, but I find just the oil and garlic with a good quality food is really all you need. could throw them some good raw meaty bones for treats and they get added calcium out of those as well.