Learn Portuguese (Portugal): A viagem não foi fácil.

You're exactly right! The verbs ser (to be) and ir (to go) share the exact same forms in the simple past tense. Foi can mean "it was" or "it went" (as well as he/she went/was). You have to use the context to tell them apart. Here, because it's followed by the describing word fácil (easy), it means "was". If it were followed by a destination, like foi para Lisboa (he went to Lisbon), it would mean "went".

Viagem (trip) doesn't end in "a" or "o". How do we know to use the feminine word a (the) before it?

In Portuguese, nouns (naming words) that end in -agem are almost always feminine! Because of this rule, you will use a (the) or uma (a/an) with them. Other common examples you might see are a mensagem (the message) and a paragem (the stop).

Why does the word não (not) come before the verb instead of after it?

In Portuguese, the word não almost always goes directly before the verb it is negating. While in English we say "was not", in Portuguese it translates literally as "not was" (não foi).

Would the word fácil (easy) change if the subject was a masculine word instead of a feminine one?