Denim is one of the most versatile essentials in a woman's closet. A nice pair of jeans will get you through anything. Besides, they are always a safe option when you don't know what to wear in the morning.
If you're in a thrifty mode, your best bet is these Asos Jeans. It's a great piece, while not very costly. Another strong contender for your wardrobe is these Asos Slim Jeans. While the price of this piece is competitive, it will surely attract a lot of attention.