Der Beginn der Reise ist immer spannend.

Because Beginn is a masculine noun in German, so its nominative singular article is der.

• der Beginn = the beginning
• grammatical gender in German is something you usually have to learn along with the noun

A good habit is to memorize nouns with their article:

• der Beginn
• die Reise
• das Ende

Because Reise is in the genitive case here.

The phrase der Beginn der Reise means the beginning of the journey/trip. In German, possession or of-relationships are often shown with the genitive:

• der Beginn der Reise = the beginning of the trip
• das Ende des Films = the end of the film
• die Farbe des Autos = the color of the car

Since Reise is a feminine noun, its genitive singular article is der.

So:

• nominative: die Reise
• genitive: der Reise

No. Here, der Reise is singular genitive, not plural.

That can be confusing because der can mean different things in different cases.

For die Reise:

• nominative singular: die Reise
• accusative singular: die Reise
• dative singular: der Reise
• genitive singular: der Reise

So in this sentence, der Reise means of the trip/journey, not the trips.

Because German main clauses usually follow the verb-second rule.

In Der Beginn der Reise ist immer spannend:

1. Der Beginn der Reise = first element
2. ist = second element
3. immer spannend = the rest

Even if the first element is long, it still counts as one unit. So the finite verb still comes second.

For example:

• Der Beginn der Reise ist immer spannend.
• Heute ist der Beginn der Reise spannend.

In both sentences, the finite verb is in position 2.

Because immer is an adverb modifying the statement is exciting.

So:

• ist spannend = is exciting
• ist immer spannend = is always exciting

This is a normal placement in German. The adverb often comes before the adjective or participle it modifies in this kind of sentence.

Because spannend is being used predicatively, not attributively.

After verbs like sein, werden, and bleiben, adjectives usually do not take endings in German.

So:

• Der Beginn ist spannend.
  Here spannend stays unchanged.

Compare that with an attributive adjective before a noun:

• ein spannender Beginn
• der spannende Beginn

There, the adjective does take an ending.

Here, spannend is an adjective meaning exciting.

It comes from the verb spannen, but in this sentence you should think of it simply as an adjective.

Examples:

• Der Film ist spannend. = The movie is exciting.
• Das Buch ist spannend. = The book is exciting.

So in your sentence:

• Der Beginn der Reise ist immer spannend.
• The beginning of the journey is always exciting.

Yes, often you could, but the phrasing changes slightly.

• der Beginn der Reise = the beginning of the journey
• am Anfang der Reise = at the beginning of the journey

The first is a noun phrase naming the beginning as a thing or stage.
The second is more of a time expression.

Examples:

• Der Beginn der Reise ist spannend.
• Am Anfang der Reise ist es spannend.

Both can work, but they are structured differently.

These words can overlap, but they are not identical.

• Reise = journey, trip, travel
  A general word for a trip, often longer or more substantial.
• Fahrt = ride, drive, trip by vehicle
  Often emphasizes the movement or transport.
• Trip = trip
  This is a borrowed word and sounds a bit more informal or modern in some contexts.

In your sentence, Reise is a very natural choice.

Yes, that is a very natural literal translation.

Possible English versions include:

• The beginning of the trip is always exciting.
• The start of the journey is always exciting.
• The beginning of a trip is always exciting.
  This last one is a bit more general in tone, even though the German sentence uses the beginning of the trip.

You can move elements around in German, but the finite verb still stays in second position in a main clause.

For example:

• Immer ist der Beginn der Reise spannend.

This is grammatically possible, but it sounds more marked or literary. The original:

• Der Beginn der Reise ist immer spannend.

is more neutral and natural for most situations.

Beginn is capitalized because all German nouns are capitalized.

So in the sentence:

• Der
• Beginn
• der
• Reise
• ist
• immer
• spannend

the nouns are Beginn and Reise, so they get capital letters.

Approximate pronunciation:

• Beginn sounds roughly like buh-GIN
• stress is on the second syllable

That stress pattern is important, because be- is usually unstressed in German.