oracle - SQL issue; count + display -

after searching more 2 hours on internet have not found specific answer questions :

1) how display 3 columns of specific table ?
2) how count (the sum) of column in table.

if don't want display columns of table should specify names of columns want display:

select column1, column2, column3 table; 

if want add values of column:

select sum(column1) table; 

if wanted display summatory along other columns, should group columns:

select column1, column2, sum(column3) table group column1, column2; 

if want count number of rows --it works sum function--:

select count(*) table;