Celery + fennel + cucumbers is a good combination that is easily tolerated by those just starting out with juicing.
Unfortunately these are not as beneficial as the more intense dark green vegetables. Once you get used to these though you can start adding the better, but perhaps less palatable ones in.
Green LEAFY vegetables are the best to use in your vegetable juicing program. All green leafy vegetables work well. The easiest ones to use include all the lettuces including, but not restricted to:
Red leaf lettuce
Green Leaf lettuce
You can then put in some of the other similar green leafy vegetables such as spinach, kale and cabbage.
Cabbage juice is one of the most healing juices when it comes to repairing an ulcer as it is a huge source of vitamin U.
Herbs also make wonderful combinations. Parsley and cilantro are great in vegetable juice.