If Else condition

Hi all,

Does anyone have an ideas on how to use if else condition in creating and altering tables which is not inside the create procedure or function scripts?
For example:
If not exists() then
create table scripts();
else
alter table.....



thanks.

if exists( condition )
begin
/// if condition is true then statements to be executed..
end
else
begin
/// if condition is false then statements to be executed..
end