No matter your experience, do not attempt to climb Mount Agung without a guide. The routes are difficult to follow and you are likely to get lost. Avoid climbing in the wet season and especially in January and February. You are still in the tropics but Mount Agung is a high peak and the top is often cold and windy so bring appropriate clothing.
Some mild altitude sickness is possible. Understand this condition and be aware of the symptons (http://en.wikipedia.org/w...). It is important to drink more water than you think you need and to avoid the dehydrating effects of substances such as alchohol and caffeine for 24 hours before your planned ascent. At these elevations though, symptoms should only be minor.
fees/permits
Mount Agung is viewed as sacred and for that reason is off limits to climbers during important ceremonies. Your guide will know and will also be able to obtain any necessary permissions from temple administrators for which some nominal donation may be required.
Guide fees may vary between about Rp 400,000 and Rp 1,000,000 depending on how you arrange the guide, the route taken and the level of English expected. Generally it is possible to put all arrangements in place once you reach Bali, but for peace of mind you might want to make contact with guides before you leave home. Some guides speak far better English than others.