Camiseta vs. t-shirt
The difference between camiseta and t-shirt is primarily geographical. While both relate to casual tops, camiseta is the standard word in Brazil, whereas t-shirt is the common term in Portugal for the same garment.
Camiseta
A1In Brazil, camiseta is the universal word for a T-shirt. However, in Portugal, camiseta typically refers to an undershirt or a sleeveless vest worn underneath other clothing for warmth or comfort.
Eu comprei uma camiseta azul para ir à festa.
(I bought a blue T-shirt to go to the party.)
No Brasil, todo mundo usa camiseta e chinelos na praia.
(In Brazil, everyone wears a T-shirt and flip-flops at the beach.)
Está muito frio, por isso vesti uma camiseta debaixo da camisa.
(It is very cold, so I put on an undershirt under my shirt.)
Você prefere camiseta de manga longa ou curta?
(Do you prefer long-sleeved or short-sleeved T-shirts?)
T-shirt
A1This is a direct loanword from English widely used in Portugal. It refers to a standard, casual short-sleeved shirt worn as an outer layer. It is rarely used in Brazil, where camiseta is preferred.
A minha t-shirt favorita encolheu na máquina de lavar.
(My favorite T-shirt shrank in the washing machine.)
Os turistas em Lisboa compram sempre uma t-shirt de recordação.
(Tourists in Lisbon always buy a souvenir T-shirt.)
Essa t-shirt combina bem com as tuas calças.
(That T-shirt goes well with your trousers.)
Nós precisamos de encomendar mais t-shirts para o evento.
(We need to order more T-shirts for the event.)
Summary
Use camiseta if you are speaking to Brazilians or referring to an undershirt in Portugal. Use t-shirt if you are speaking to Portuguese people about casual outer wear.