私は友達でも年上の方には敬語で話しています。

It should be used with customers, co-workers with a high status than you, your manager, boss or anyone else who is "superior" to you in the work place.  If you're working at a place like a fast-food restaurant than you would use バイト敬語 instead.

Sometimes women will use it to their significant other to show that there's distance within their relationship.  Otherwise you wouldn't use it outside of the working environment.

謙譲語 is used when you lower yourself to higher someone else. Therefore if you use a pronoun it must always be "I"

Ex: 念のため、ご連絡先をうかがってもよろしいでしょうか？(Just to be sure, may I have you contact information?)- This would be very useful when getting a customer's contact info (such as phone number) to your boss.

丁寧語 is simple「～です」 and「～ます」forms that you'd use to be polite but the people you're speaking to and people speaking to you have an equal status. 

Ex: 　明日、原宿に買い物に行きます。

（尊）敬語 - You can think of this as being extremely polite and is something you would use with 謙譲語.

皆様で召し上がってください。（Please eat, everyone!)

バイト敬語- What you would use at your part-time job but it's not exactly "correct" 敬語 though but it's often heard.

Your waiter or waitress may say this to you:

お飲み物は大丈夫ですか？Would a drink be okay?　（Correct but probably not used as much: お飲み物はよろしいですか？)

Hopefully this helps and perhaps a native speaker could provide more insight as well.

ざっくりと簡単に言うとフランス語でTuの代わりにVousを使って話す時が日本語の敬語にあたります。あまり親しくない間柄や公式の場などで主につかわれます。ただ、日本の場合は親しくても相手が年上であれば敬語をつかって話すことも多々あります。

It can be used to anyone to whom you want to show your respect. Some people use it even to their family members and some don't use it even to their superiors. That being said, the majority of native speakers use it to their seniors and superiors, and not to their family members or frields. 

I would say it is safer for learners to always use it than never to use it.