T-shirts are indispensable, even if you spend ages thinking about what to wear: in the end, it can always be a T-shirt. T-shirts come in prints, plain colours, tight or loose fits. Round necks or V-necks are very popular.
For a particularly well-rounded outfit, it is a good idea to pick up a colour from the T-shirt in the lower-body items. For a more edgy look, you can of course wear a T-shirt in a bold colour and then keep the rest neutral to set off the colour perfectly.