|And you shall take for you on the first day, the
fruit of the tree "hadar" and branches of palm trees, and a bough of the tree
avot and willows of the brook, and you shall rejoice before Hashem, your G-d, seven
days. (Leviticus Chapter 23).
The mitzvah of taking the four species is for all seven days of Sukkot. Two blessings are said the first day, al netilat lulav and she'hecheeyanu and one bracha, al netilat lulav is said for the other six days. According to Torah law, the lulav is taken for all seven days only in the Holy Temple in Jerusalem when it was standing, and outside the Temple only on the first day of Sukkot. Our sages ordained, however, that we should also take the four species for all seven days as a remembrance of Temple Times.
The four species all taken together make up one single mitzvah. If any of the species are missing then you have not fulfilled the mitzvah at all. One lulav, one etrog, two aravot, and three hadasim is taken on Sukkot. The lulav, hadasim and aravot are bound together.
The four species are not taken on Shabbat even when it falls on the first day of Sukkot. The mitzvah applies during the day but not at night.
On the first day of Sukkot, a person must be careful to own the lulav and etrog he is performing the mitzvah with. On the the other six days, ownership is not strictly required.
A person should try to take four species, which are beautiful. Of course, this has to be done for the sake of the mitzvah and in service to G-d; not for the purpose of showing off to the neighbors.