Hi,
Glad to see your interest on darbuka. One of the most popular beginner doumbeks is the Alexandria Aluminum doumbek. First time players like this drum because it is a moderate size, very durable and has a good range of tones. It is a good drum to learn the basics of doumbek playing on. It also produces quality tones so that even advanced players enjoy playing it too. Also, it's cost is lower than most other doumbeks, which makes it a great drum for the price.
Other doumbeks that are a little more expensive than the Alexandria doumbeks, but are easy to play and have beautiful tones are the Copper Doumbeks (excluding the large ones) as well as the 8 inch or 10 inch Brass doumbeks. In addition, these drums are also a moderate size, which is another reason why they are good beginner drums.
As with most things, you get what you pay for. The Alexandria doumbeks are an excellent drum for the price, but the copper and brass doumbek shells are considered to produce prettier tones than the aluminum shells.
One last point, the Alexandria aluminum and the copper doumbeks have one advantage over the brass doumbeks. The advantage is that the Alexandria and copper doumbeks have curved rims and this type of rim is a little softer on the fingers & hands while playing. Nonetheless, all three styles are good beginner drums for the reasons indicated above.