So first get ready with study material.
Where to find 10g Application Server Study Material ?
For 10g Application Server check
10g Application Server Documentation, Click Here . This is documentation for Application Server 10.2.0.2 as this is the latest currently
So lets Start with Installation of 10g Application server , you can see Installation guide from above location or from Installation Guide Click Here(This is for linux version)
Where to find Oracle 10g Application Server Software ?
Software for Installing 10g Oracle Application Server is available at http://edelivery.oracle.com/
What should ne Order of Installation ?
As OAS (Oracle Application Server consist of ) consist of OAS Middle Tier & OAS Infrastructure , you should first Install Infrastructure . Why ? because Middle Tier services need Infrastructure .
Can I install Infrastructure & Middle Tier under same ORACLE_HOME ?
No they can't be installed under same ORACLE_HOME they need separate ORACLE_HOME but they can be on single machine or separate machine(Your wish)
What in Installed under Infrastructure ORACLE_HOME ?
Under Infrastructure oracle_home you will have
database (which will hold data both configuration data & content data)
Oracle Internet Directory directory for user credential & other application information. (This inturn consist of DIP & replication oid will explain later )
Single Sign On (consisting on HTTP_Server, OC4J_Security)
Oracle Certifying Authority (Optional, will explain later don't install it at this minute)
High Availability (Optional, will explain later don't install it at this minute)
What in Installed under Middle ORACLE_HOME ?
depending upon type of MT Installation you selected you will see
WebCache (Cache to store information for better performance as well other uses like load balancing, reverse proxy ..)
J2EE (for java application)
HTTP_Server ( Oracle-Apache Webserver for accessing application over internet)
So go through installation Guide mentioned above & put your comments if you have any doubts .