If the summer is usually judged on the amount of clothes we shed, autumn/fall is always viewed as the return to a more clothed existence!
A shirt is the basic form of attire to jumpstart the process of dealing with the awareness that our warm-bloodedness only works up to a point (in the summer, when societal pressures/personal inhibitions make sure we have something on, a shirt or t-shirt is usually the thing that keeps nosy, prudish people from rioting).
Over here, the basic term for shirt is [a] camisa, which is usually set aside for formal-looking, buttoned shirts; loose, usually short-sleeved shirts without buttons are known as [a] camisola¹ (de manga curta), with the term [a] t-shirt (t pronounced just like in English, tee) also being quite popular.
¹ Weirdly enough, the term camisola is so broad that people can also use it for wool or knitted shirts (which are usually long-sleeved); T-shirts are therefore called [a] camisola de manga curta (short-sleeved shirts) if that disambiguation is necessary.
In Brazil, the word used is [a] camiseta. Like most differences, it is something we’re used to hearing (due to Brazilian telenovelas and the strong Brazilian community that lives over here) – and the Brazilian nationals who live in Portugal eventually also become acquainted with the different words we use, even if some of them choose not to use them in their daily lives -, but it’s always best to know which terms go with each variant to make sure you’re perfectly understood.
Knowing the different words and using them differently according to whom you’re speaking can also be seen as a sign of versatility and politesse, but it’s not altogether necessary if you can tell the person will understand you regardless (as I mentioned many times before, this works better in Portugal than in Brazil, where most locals don’t have a thorough knowledge of differences between BP and EP to fully understand all your EP words).