The basic tenets of Wicca are harm none, reincarnation, reverence for nature, personal responsibility, the rule of three, and celebrating the old religion. Many groups list thirteen to twenty tenets, but each can be boiled down to these six concepts.
The Wiccan Rede is a moral code that basically says not to harm others. A common interpretation is, "An it harm none, do what ye will." It means to go after the things and experiences one wants in their life, but not to do things - magically or mundanely - to harm others in the process. Some feel the Rede means to be a vegetarian. Others think the Rede means no casting darker magick spells. Still others interpret it to mean not to use others or be manipulative to get what they want. It's open to what the heart hears, or what one's coven (think Wiccan Church) interprets. Needless to say, most Wiccans don't partake in animal sacrifice, virgin sacrifice, children sacrifice, black magic, etc.
Most Wiccans believe in one form or another of reincarnation. Again, it can be interpreted in many ways. Some believe we are born back into our own family line, making the ancestors become the descendents. Some believe we can only reincarnate so many times, and then we go to a final resting place called the Summerland. Others say a soul can reincarnate an infinite number of times.
Wiccans have a strong reverence for nature, and many practice their craft outdoors so they can be amidst the trees or under the moonlight. Others celebrate their respect for nature by caring for pets and plants, or by learning Herbalism. Most Wiccans view the Earth itself as a living, breathing goddess. Many Wiccans care about recycling and green options so they can be better stewards of the planet. The holidays, aka Sabbats, found on the Wheel of the Year all venerate the cycle of nature.
Wiccans believe in personal responsibility. They don't blame their actions on devils or demons or drugs. They own the choices they've made,the actions they've carried out, and the words they've spoken. Right or wrong, a Wiccan wants to learn and grow from their experiences, and owning their behavior and being personally responsible is how they achieve it.
The Rule of Three is one of the basic tenets of Wicca most Wiccans believe in. It says that anything you do - good or bad - will come back into your life multiplied by three. If you are selfish, you will experience people being selfish with you. If you are kind, you will be treated with even more kindness. If you send healing energy out through a spell, your ailments will also be healed. If you are nasty to someone, people will be short with you.
Wiccans celebrate the pre-Christian religions that can be found in mythology books of the Greeks, Romans, Celts, Norse, etc. The gods and goddesses of the myths are revered. The tenets of the old religion are adhered to. The myths themselves are studied and meditated on to better understand their meaning and hidden truths. Wiccans believe that, even if today's rites aren't carried out exactly the way their spiritual ancestors carried out rites, that their spiritual ancestors would still recognize what they are doing, which is breathing new life into the old religion.