Decent guide from Mashable if you're interested in making your own cocktails and how to go about it. The video isn't so helpful but the writing is.
For me the most common mistake people make with cocktails is overcomplicating it. Over the years, classic cocktail recipies have been bastardised and new cocktails seem to feel the need to have about ten different ingredients, making it very tempting to go crazy for those just starting out.
Start by limiting yourself to drinks with 4 or less core ingredients. Not only will you have a greater chance of success by doing this but complex, modern-day cocktails are generally overrated anyway. Cocktails should be simple, the whole idea is to combine a select few ingredients that compliment each other well and add up to something greater than the sum of their parts. If you're adding tons of stuff together then you're trying too hard to force something that probably isn't there.
Take for instance my favourite cocktail: a vodka gimlet. Vodka, lime juice, and simple syrup – that's all it is. Why not give it a go?