I juice just about everything. I try to keep away from using a lot of fruits because they are high in sugar. But yes, you need something to cut the taste depending on what veggies you use. I either use an apple, a lemon (for very green juices), and oranges for certain juices. Grapes give a lot of sweetness too.
I almost always use three stalks of celery, but I guess I'm used to it. I like the slightly salty taste it gives. I also always add a leaf or two of kale. Remember, you don't need a ton of any specific veggie. I'll add a leaf or two of cabbage too. If you add a little, you probably won't even taste it but you'll still reap the benefits of it. Just don't overdo any one green leafy veggie.
Also, some veggies give more juice than others. Half a cucumber gives a lot more juice than a bunch of parsley. So you kinda play around with the amounts and you'll eventually get the hang of it.
Some of my fave recipes are:
2 oranges, 4 carrots, and half a peeled sweet potato.
spinach, cucumber, celery, lemon, and ginger.
one small beet, 3 celery, and two apples with an inch of ginger.
depending on your juicer, you won't need to peel or cut much. I always cut the ends off carrots, peel any sweet potato, and ALWAYS peel the orange. But I throw the entire lemon in, entire apple, celery with the leaves and all. All the peels have a lot of nutrients in the juice, so try to keep as much as you can.
When I started juicing, I wrote everything down in a "juice journal". After tasting the juice, I'd make notes in it. That way I knew what was good and what was baaaaadd. I've only made one real bad juice. It was mostly oranges and tomato...HOLY HEARTBURN! Even if some don't taste as good as others, I always remind myself how good they make me FEEL.