Do I learn Oracle 11g or 12c?

Both. Learn 12 - most of what you'll learn will also apply to 11g systems. 11gR2 is ten years old. Get ahead of the curve and learn 12c, you'll be a stronger candidate.

It really doesn't matter. If you have close to zero Oracle knowledge, start with learning concepts. (There actually is a piece in the Oracle documentation called concepts, which is esential reading for anyone trying to get into Oracle DB world). You do not need to pay much attention to versions while learning, those features that make the difference are very expensive and it is not very likely that you'd encounter those while you're getting into the game. Once you have a decent understanding of the concepts and architecture, then start learning whatever will be current version at that time. These days it's 12c.

I've worked as an Oracle DBA since the product only ran in PDP-11 compatibility mode and Larry Ellison still made sales calls. Your number one difficulty in learning Oracle is the language barrier. Oracle calls everything they do something other than what a rest of the industry calls it. For example, a deployed database is called an "instance". And it just gets waaaay worse from there. So the first thing you need to do is learn the new Oracle language to make your life easier. Your SQL server experience will help and hurt you at the same time. You'll know how easy something is to do with SQL Server (Sybase) and how frustrating it is to do with Oracle. Learn whatever release your shop is gonna be using first. Good Luck, Bro!

Oracle customers upgrade on their own schedule. Some companies are on very old releases just because they lack the resources to validate their software on the new release. Others upgrade the release, but never take advantage of the newer features. Therefore, the answer to that part of your question is "it depends". As for which to study, I'd never recommend getting a certification in an older release just because the value of it on your resume tends to be more limited. Personally, I'd go for the 12c training. You will get the essentials of the engine and you will be aware of the latest features which might be useful should the conversation come up as to what could be done if an upgrade path was elected. Meanwhile, your resume has a longer shelf-life. Good luck.

from programming perspective- plsql programs, sql scripts, 11g plsql features to a great extent is same in 12c. while 11g was created for scalability using grid architecture, 12c was created for speed and scalability mainly for organizations with large data centers with cloud architecture. it will be beneficial to know both versions especially during database upgrade, that will help you provide advice business/IT team on obsolete features or if there is need for some additional infrastructure for certain features to work.

Well Oracle 12c is just the latest release. It encompasses all features from previous releases.  So learning 12c is a better choice. Then again you did not say at what level / for what tasks you need to acquire Oracle knowledge. Is it to develop code that uses an existing database ? Or is it do design a database, logically and/or physically ? Or is it to operate an existing Oracle environment (essentially being a DBA). Those are different profiles with very different needs.  For example the major new feature in 12c, multi-tenant environments is only of concern for admins that deal with many databases, and DBAs must know how to manage that. A developer does not care much - if at all.

I would go with 12c as it's the newest one out there and a lot of the concepts translate back to 11g. Throughout my education I took classes that used 12c, and now that I work on systems that leverage both 11g and 12c databases learning the newer technology definitely helped.The biggest difference in 12c is oracle's multi tenant environment setup, which is mainly to address hosting in the cloud. There are several other improvements, which I would imagine a DBA might be able to shed more light on.

I would go with the latest.   There are still shops that have older versions running, but anything current and exciting is likely to involve the latest version, or an upgrade to that version very soon.   Best advice - read and commit to memory the "Server Concepts" manual.    You will find you'll know more than most.