I'm not a painting expert, but here's what learned when I recently re-painted all the inside window frames in my house when I replaced the windows:
Trim is usually done in white high-gloss paint. One thing I discovered is that you will probably want to try to match the white to existing trim in the room, unless you want to re-paint all the other trim in the room to match--- in entryways--door frames, etc. and baseboards, too.
All white is not the same! Some white is bluish, other has a yellow cast, some brown or gray or even pink. (just look at the white paint color strips at the store! It will drive you nuts!) To match your existing color, try taking a fingernail-sized chip off the mantel from a hidden area ( like near the floor on the side--less visible=less sanding) Most paint places can match the background hue of your white from there.
I learned this in the paint department at Lowe's: There's a product called (something like) "liquid sandpaper" that will prep the surface before you paint. which is great if you have any dentil molding, etc. or other fine details which would be hard to get at with sandpaper. It's also a lot less mess and dust. PROTECT YOUR FLOORS and hands, and use safety glasses with toxic or corrosive chemicals, please!
I was told this is very important: You can paint oil-based over latex, but not latex over oil-based paint. I'm not sure how to tell the difference, but I'm sure someone in the store can help you with this. One clue will be the age of your home. Ours was built in '89, and oil was still in use. A knowledgeable person in the paint dept. will probably be able to tell you from your paint piece if it's oil-based or not--then you can go from there.
As to the gloss level, what is on the other trim surfaces in the room? Is it the largest and shiniest piece in the room, and you're looking to tone it down a little? I'm not a decorator, but I wouldn't think you have to go with the highest gloss finish for the mantel--nothing super-shiny. You definitely want gloss for trim, but it doesn't have to be the glossiest. Again, ask for help in the paint dept. You'll be amazed at how helpful these folks are! I was, and incredibly relieved, too. Good luck.