Simple: you can believe in the existence of many gods and goddesses... doesn't mean you have to worship them all. If you like, think of Yahweh (or Jehova - whatever you chose to call him) as the God of the Christians, just one of many gods, not the ONLY god. He has his own mythologies (in the Bible) and is part of his own particular pantheon (The Trinity at the top, the various levels of archangels and angles, the Devil and the demons, the Saints etc). If you can believe in God and the Devil at the same time - you can believe in multiple deities - just a different "family" of Gods if you will.
Say you were living in a society whose culture contained a pantheon of gods. If you were a blacksmith, you would probably worship the God (or Goddess) of blacksmithing (depending on the pantheon), but that would not necessarily mean that you didn't believe in the existence of any of the other gods in that pantheon eg. a Goddess of Love, or a God of War etc.
You can BELIEVE in the existence of a God, you can have FAITH that that God will act on your behalf (or listen to you, or have a certain remit), you can actively WORSHIP that God. You don't necessarily have to have or do all or any of the three.
Where it does start to get a little confusing, is that the God you were worshipping before is A god called Yahweh (whom many of his followers say is the ONLY god). The God and Goddess can be a pair of archetypes representing the masculine and feminine divine principles or certain aspects such as light and dark, mother and father etc., they can also be generic names (or terms) for two (essentially nameless) opposite and/or complimentary deities. They can also be a specific named God and a specific named Goddess from a particular pantheon with whom you feel affinity.
Not every pagan worships a pair of deities (or any deities at all), some acknowledge the existence of many but only worship one. They may have been claimed by Brighid, but still believe in the existence of Lugh, or the Morrighan. They may be claimed by Odhinn but still acknowledge the existence of Freya, Thor, Loki etc.
Wrapping your head around the existence of multiple deities can be hard. You would probably be well served by getting yourself a book on mythology and reading some of the myths and legends surrounding different gods from different pantheons.
Wicca (as in British Traditional Wicca - as defined on this site) works with a specific pairing of God/Goddess, whose names and rites are only known to the initiates of that (those) particular tradition(s). So wrap your head around the multiple deity concept and non-Christian mythologies first.
This might help you get your head around the various views of the divine in the pagan community:
Glad it helped you. A friend of mine wrote a brilliant essay / workshop many years ago called "Knowledge Of The Gods" and in it she used the example of a God of Blacksmiths. I always thought that was a very succinct way of explaining it.
For many pagans that I have spoken too, they ceased to believe in, or acknowledge the existence of, the Christian God long before they found their pagan paths. I personally never had to wrestle with the dilemma of trying to believe in many gods when I still believed in ONE god. Hence trying to give you some examples of how you can believe in both. I studied Ancient History at school and always wanted to worship the Greek Pantheon (I was at a Catholic School), but didn't realise that one actually could.
So when I found out that people did actually acknowledge and work with the Old Gods, it wasn't such a leap to go there myself.
One thing that I thought I would mention, is that to mix your pantheons isn't always a good idea (not always a bad idea, but sometimes it's NOT a good idea either.)
One thing that I see many people new to paganism do, is step half-way in... That is, try some pagan practices or similar, but do so with the Christian God..
In most cases I see this as sitting on a fence.. Someone who feels that it's somehow "wrong" to do what they want to do, usually due to years of ingrained beliefs, and so goes half-way... As if to say "Well I'm not a REAL witch, because I use Jesus in my beliefs.. See? I won't go to hell, this way," and such. Don't get me wrong, but it IS what I see time and again, and almost always by those very new to paganism.
One other thing to consider in your own workings, is whether your god and goddess would get along, or be insulted in being put together, in my opinion.
If it helps, you can think about belief in Deity like learning styles. For example, I'm currently in school for massage therapy, and in my study group we have two people who are auditory learners and learn best through hearing and talking about the material, one person who's a visual learner and learns best by reading the chapters and looking at the pictures, and one person who's a tactile learner and learns best by re-writing all of the information and making flashcards for the rest of us. But in the end, despite our different approaches, we're all learning the same information.
Similarly, you can think of the various gods and goddesses as all being different ways of understanding the one Deity Force (or whatever you want to call it) that's out there. If everyone has different learning styles, it's ludicrous to think we'd all relate to Deity in the same fashion (if at all), so multiple approaches makes sense. Hence, Yahweh and Allah and whoever else are all just different personifications of the same Deity, so really it doesn't matter who you worship b/c all worship is ultimately directed at the same Deity. I know a lot of Pagans would disagree with that, but it's what I've always believed (aside from my time as an atheist, at any rate).