First you need to get to Berastagi. An air conditioned minibus from Medan costed Rp35,000 in 2001 (about $3.5), but prices can
change quickly in Indonesia. If you are in south Sumatra you can take a minibus to Lake Toba (Parapat) and change there.
The guesthouse in Berastagi you want is Wisma Sibayak. It's a really nice place with good food, low prices and very good service. Just tell the driver to drop you off there. He'll know where it is.
Read all the comments earlier visitors have left in the comment books. Get a map. Wisma Sibayak have one and Elshaddai Guesthouse have another. They're free. Ignore all the notes on the walls listing people who have died trying to climb the volcano and encourage you to get a guide.
Most of the lost trekkers tried to climb Gunung Sinabung, which is much harder.
But don't underestimate the volcano. If clouds roll in things can get difficult.
The perfect time to climb is on a Sunday morning, because all local youths will have climbed the volcano the previous day to play
guitar, drink beer and have sex (I'm just guessing here). They sleep in tents and climb down in the morning, so you'll know you're on the right way if you meet local youths. Say "selamat pagi" (good morning).
If you follow your map you won't have any problem getting to the summit
(even if you're not climbing on a Sunday morning). When you get to the place where there are 3 blocks of stone in the middle of
the road, climb the little path on your left. Soon after you'll find a set of concrete steps which, even though parts are missing, will make the path easy to follow.
After about 2 hours of climbing totally, you should now have reached the crater. Not very impressive, is it? And it smells terrible too (sulphur). Do climb up to the radio antenna. You want to have the best view possible, right?
Finding the path down from the volcano is the tricky part (unless you just take the same way down). Continue on the path you were coming from and turn 90 degrees right when you reach the crater. Climb up to the rim and, if you don't see the path down immeditely, walk along it in an anticlockwise direction until you do. Enjoy the view (unless there are clouds in the way).
The path down is quite easy to follow. After an hour and half or so of descent you should reach the little village below. Have a nice, soothing bath in the hot springs there. You deserve it.
Congratulations! You have just climbed what is possibly the most accessible volcano in the whole of Indonesia!