So first of all get yourself these things
Leg of lamb you can use a shoulder too (I usually use 1kg leg for 4 people, ask your butcher how much more you might need).
3-4 stalks of rosemary
sea salt and freshly ground cracked pepper
4 cloves garlic cut into quarter sized pieces
half glass red wine (cooking wine ie cheap wine is what I use)
Vegies for roasting (ie potato, pumpkin-butternut squash is best, sweet potato, carrot, parsnip)
Preheat oven to around 220c (around 430 F) (fan forced, so if its not fan forced, make it a few degrees warmer).
Cut little slits into the lamb a few cms apart, all over the top of the lamb and at the sides (and if there is a shank, do it there too).
Into these slits place garlic and rosemary sprigs (so break small sprigs off the stalk).
Season the lamb with salt and pepper and drizzle over some olive oil. Then pour over roughly half a glass of red wine.
Place in oven, you can turn it down to around 200c now (if not fan forced make it a few degrees warmer). Meanwhile peel any vegetables you want to roast, like potato, pumpkin, sweet potato, carrots, parsnips etc. I usually place the potatoes (that have olive oil, salt and pepper and some left over rosemary thrown on top) into the oven about 45 mins after the roast goes in, and then after about 10-15 mins I put the other vegies in with the potatoes. Some people place it all in one dish (ie lamb and vegies) but I don't, sometimes the vegies aren't done, or are done too quick).
After about an hour of cooking the lamb, take it out and baste it again with the juices that have formed in the pan (if it has all dried up, add a little water to the pan and a little wine and then put that over it).
The lamb this size takes around 1.5-2 hours to cook. You can take it out around 1.5 hours and check how its going by just cutting a small slice through the side, if its still quite rare then just leave it for say 20 mins and then check again. If you find the top is burning, you can place some foil over the top before you put it back in. You take it out when the meat has a slight pink colour to it, only slight, unless you like your meat rare then take it out when its a little pinker. Take it out of the pan and let it rest on a cutting board. The vegies should now also be done.
While its resting make up some gravy. I use a powder gravy max and pour in the juices from the pan, you can make your own gravy though, there are loads of different ways to do it on the net.
Then carve the meat, no real way to do it, I don't like it really thick, so I cut mine thin, usually you cut away from you. And you're done!! Now dig in before it gets cold!!