Enabling AWE on SQL SERVER 2008
The server has 32 GB Memory and we assigned 22 GB to SQL Server but its using only 1.5 GB


This is because when 32 bit SQL Server is installed on 32 bit Windows Servers it can see maximum Virtual Address Space(VAS) of 2 GB so its memory consumption under default behavior cannot be more than 2 GB no matter how much RAM is present on windows server.


  Do we have to enable\configure any options on Windows before enabling the option on SQL Server?


As I can see you want SQL Server to use more memory and for that you are enabling AWE which is correct but first you have to make sure windows server can see more than 4 G of RAM. Again maximum VAS for 32 bit windows system is 2^32 which is 4 GB so any OS process cannot see more than 4 G of RAM. The good thing is you are having Window server 2008 enterprise and you can enable PAE so that windows server can see upto 64 G memory.

To explicitly enable PAE, use the following BCDEdit /set command to set the pae boot entry option: Take windows team help to make the changes.

bcdedit /set [{ID}] pae ForceEnable


IF DEP is enabled, PAE cannot be disabled. Use the following BCDEdit /set commands to disable both DEP and PAE:

bcdedit /set [{ID}] nx AlwaysOff
bcdedit /set [{ID}] pae ForceDisable


Once PAE is enabled windows server can see more than 4 G and then you can enable AWE so the SQL Server data and index pages can see memory more than 2 GB.


  How much memory can be assigned to SQL Server on the Enterprise Windows (Max)?


For 32 bit, maximum memory like I said is 2GB. But when you enable AWE on windows server which can see more than 4 G, ONLY SQL Server data and index pages can use this extra memory not other caches like proc, plan etc.


  Is there any other thing we should take care of before applying since its a production server?


Just test enabling PAE on UAT, enabling AWE is pretty much straight forward and easy.

I suggest you read 


SQL Server memory and troubleshooting
What is difference between VAS and Buffer Pool
Understanding VAS reservation in SQL Server


PS:  Please upgrade to 64 bit ASAP. This will save you lot of hassle moving forward. SQL Server 2016 is now only for 64 bit system so you can see MS is moving away from 32 bit

PPS: SQL Server 2008 is patched to SP2 please patch it to SP4 ASAP.