I was given a juicer by a friend of mine and am wondering if anyone can suggest effective combinations of fruit/vegetables to use.
How many times a day should i juice and is there anything to avoid ?

I don't understand the question of how many times a day you should juice. If you want juice, then make it and drink it. I would not juice as a meal replacement. And I would use juice in the place of fresh veggies and fruit because when you juice you remove the fiber.

As far as combos it really depends on what you like. Some common ones are Apple with carrot/celery/cucumber (use all or one of the veggies).
Pear with the same.
I like Beet and apple.
I've put lemon and ginger in my apple/carrot juice.