The pre-requisites depend on which export/import method you are planning to use.
Will it be datapump or will it be the legacy export/import utilities?
We are going to use Datapump.
What are the pre-requisites we need to do on source database before export?
Like reindex, gather statistics etc...
There is no as such hard and fast rules. It depends what type of feature you are going to use . Are you going to use PDBs is your New 12c DB is a CDB. Then you will have issues while importing the users in 12c as in 12c Common users are created with C## so in that case you have to use a PDB.
Could you please help me what are the pre-requisites and steps to export 11g database and import in 12c database?
The Oracle docs can tell you that.
Have you read them? If so post a link to the doc you used and a quote from a section you don't understand.
What version of 11 are you coming from. The real version like 22.214.171.124.0 or 126.96.36.199.0. Something like 11G doesn't tell us the real version. Also, what are you doing with the migration? Are you changing hardware? changing disk storage types?, etc. There is a 500+ slide deck that the upgrade/migration team that talks about migrations, but I'm sure the majority of it is not needed for your particular case.
Gathering stats would help in the queries that Data Pump will use.
Excluding stats, if you don't mind regathering them on the target, would speed up the export and then the import jobs.
Obvious taking the export when you can stop all of the users from making changes would be a good time to run export.
Just too many unknowns to help much.
Here is a good place to start: https://blogs.oracle.com/UPGRADE/entry/oow_2015_upgrade_and_migration
Hope this helps