Casting spells and participating in rituals is often a major part of a Wiccan's practices. To cast a spell or conduct a ritual without proper respect would be as wrong and as blasphemous as yelling during a Catholic Mass. Before even start to think about casting a spell, you should first research all that you can. Make certain your spell isn't harming others or yourself. Respect should always be shown, even if Wiccan ways are not your own.
If a Witch you know and feel comfortable with, invites you to cast a spell or participate in a ritual with him/her, you may do so. If you do not wish to participate, you don't have to. Wiccans never pressure others to worship as they do. Nor do they preach to others. Again, be respectful at all times. Also, remember to ask as many questions as you want before the ritual or spell begins! Wiccans are always happy to teach others. Learning is the best way to battle fear caused by ignorance.
Should you wish to participate in Wicca more fully, think about undergoing a Dedication Ritual. A Dedication is when a person states that they wish to be acknowledged by the God and Goddess as learning seriously about Wicca. A 'trail period' of a year and a day, is common. A Dedication does not make one a Witch: it makes one a serious student.