Hi
I'm Pagan, and have followed a pagan path for more decades than I care to admit.
To answer your questions:
- what are the main beliefs/practices
Paganism is an umbrella term, a little like Hindu or Christianity, in that it covers a myriad of beliefs. There aren't even any central tenets, though most Pagans accept that Divinity can have both male or female aspects. How individual pagans decide to honour Divinity is also very personal. Popular paths within Paganism are Northern/Heathen paths (think Thor/Vikings, etc.), Wicca (an initiatory religion founded in the mid-20th century by a man called Gerald Gardner), and Druidry.
- who are the main gods / goddesses
There are hundreds, depending on an individual's path. Some pagans are polytheist and will worship/honour specific deities, such as the Greek or Norse pantheon. Others will simply honour male/female aspects of divinity, such as "the Horned God" or "the Mother Goddess".
- how is it connected to wiccanism?
There are two "wiccas". The first, and original, is the mystery initiatory religion founded by Gerald Gardner, which requires initiation into a lineaged group/coven which can trace it's initiatory line back to Gardner. There are also specific things which are common to all Wiccan groups with this lineage. However, the term Wicca was also adopted as a publicity-friendly title by publishers and authors wanting to sell books, so the idea that people could read a book, follow the ideas within that book, and call themselves Wiccan, also got out into the public sphere. And "wiccanism" isn't actually a word 
- how does it feel about Christianity
Some Pagans like to feel persecuted and will be convinced that Christians are continually harassing them. (Actually in America this is probably true and some pagans are in fear of being exposed as they could lose their jobs/homes). But most pagans in the UK are quite sensible and it's very much live-and-let-live. Many of my pagan friends will have a good old sing-song at Christmas too.
The BBC website is quite a good source of general information, or the Pagan Federation.