As a newbie myself, I can only give you advice based on what happened to me:
Check each animal you buy. Make sure it has enough teeth on the bottom of it's jaw. Don't just let the seller tell you it's OK and find you have a ewe with a dodgy leg and no teeth (as I did yesterday).
Every breed seems to have it's pro's and con's. We work every day, so wanted sheep that look after themselves. We went for Welsh Mountain breeds. Pro's: Great feet, easy outdoor lambing, hardy, lambs get straight up. Con's: Not really "tame", can jump over sheep hurdles, can jump over a 4' stream, eats trees, eats hedge, eats anything!
Keep an eye on them when you first get them. Ours weren't treated with a preventative for Fly-strike, so one went down with it within days of arriving. We then, of course, treated all our sheep.
Sheep will die. Find out who you need to contact before it happen's. Don't end up fretting over an entire weekend because no-one answers the phone!
That's what I've learnt since November