Dreh den Lenker um dreißig Grad.

Because this sentence is a command addressed to one person in the informal du form.

The verb is drehen.
For the du-imperative, German usually drops the -st ending and also leaves out the pronoun du:

• du drehst = you turn
• Dreh! = turn!

So Dreh den Lenker ... means Turn the handlebar/steering control ...

German commands often start directly with the verb.

Yes. For many verbs, the du-imperative can appear both:

• Dreh den Lenker ...
• Drehe den Lenker ...

The version without -e is more common in everyday German, so Dreh sounds very natural.

Because Lenker is the direct object of the verb drehen, so it takes the accusative case.

The noun is:

• der Lenker = nominative
• den Lenker = accusative

So:

• Der Lenker ist kaputt. = The handlebar/steering control is broken.
• Dreh den Lenker. = Turn the handlebar/steering control.

Since Lenker is masculine, der changes to den in the accusative.

Lenker is something used for steering. Depending on context, it can mean things like:

• handlebar on a bicycle or motorcycle
• sometimes a steering control
• in some contexts, something like a steering wheel is possible, though Lenkrad is the usual word for steering wheel

So the exact English word depends on what is being controlled. In a bike context, handlebar is the most likely meaning.

Here um means by in the sense of by an amount of rotation/change.

So:

• um dreißig Grad = by thirty degrees

This does not mean the final position is thirty degrees. It means you rotate it through thirty degrees.

Compare:

• Dreh den Lenker um dreißig Grad. = Turn the handlebar by 30 degrees.
• Dreh den Lenker auf dreißig Grad. = Turn it to 30 degrees.
  This is less natural in many contexts, but it shows the difference between by and to.

In standard German spelling used in Germany and Austria, thirty is written dreißig.

A few useful notes:

• ß is called Eszett or scharfes S
• In Switzerland, people normally write dreissig instead, because Swiss Standard German does not use ß
• The word is pronounced roughly like DRY-sig, not like English three-zig

So dreißig is the normal spelling for most learners of standard German.

In measurement expressions, German very often uses the unit word without a visible plural ending after a number.

So you say:

• ein Grad = one degree
• zwei Grad = two degrees
• dreißig Grad = thirty degrees

Even though English uses degrees, German normally keeps Grad in this kind of expression.

Because in a German imperative sentence, the verb normally comes first.

So the basic structure here is:

• Dreh = verb/command
• den Lenker = object
• um dreißig Grad = phrase showing by how much

This is very typical German command word order:

• Öffne die Tür. = Open the door.
• Nimm das Buch. = Take the book.
• Dreh den Lenker um dreißig Grad.

Because they are nouns, and in German all nouns are capitalized.

So:

• Lenker = noun
• Grad = noun

This is one of the most noticeable spelling rules in German.

It is informal singular: you are telling one person that you would call du to turn it.

Other versions would be:

• Dreh den Lenker um dreißig Grad. = informal singular
• Dreht den Lenker um dreißig Grad. = informal plural
• Drehen Sie den Lenker um dreißig Grad. = formal singular or plural

So the exact form of the verb tells you who is being addressed.