What is cluster and noncluster index in sqlserver why we use and how to use it ?

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SQL-Server-2008R2

hello guys i am stuck in cluster and non cluster index i am read different articals about it but still i am not clear on it please help me to clear it

i have create one table like

create table Student (SId int not null identity(1,1) primary key,Firstname nvarchar(50),Lastname nvarchar(50),Gender nvarchar(6),MobileNo decimal(11,0),Address nvarchar(300),City nvarchar(50),DOB datetime, created DATETIME not null DEFAULT(getdate()));

What I have tried:

i have a Student Table i have wrote query for cluster and non cluster index given below

create clustered index Student_ind on Student(Firstname);

when i am run above query i have shown a error :Cannot create more than one clustered index on table 'Student'. Drop the existing clustered index 'PK__Student__CA195950D791869D' before creating another.

but i have create cluster first time then why this error occur

create index Student_no_ind on Student(created)

for execution i have used this command
exec SP_HELP Student

i have get output also but i dont understand it what kind of output it is