In English, there is only one way to say : 'to go TO + country'.

But in French, it is more complicated. Let me explain you how it works.

I - For the city

Before a name of the city, most of the time it is 'à' .

But if the name of the city start by 'le', we use 'au' and by 'les' : 'aux' .

<em>Example :</em>

Le Havre = je vais au Havre

Les Ulis = j’habite aux Ulis

Demain, je vais à Paris    <em>Tomorrow, I go to Paris</em>

Ma sœur vit à Bruxelles   <em>My sister leaves in Bruxels</em>

Ma meilleure amie réside à Dublin  <em>My best friend leaves in Dublin</em>

II - For the country
1. If the country is a plurial name ('les') we use 'aux'
Example :

les Etats-Unis = je vais aux Etats-Unis  

2. If the country is singular and masculine ('le') we use 'au'

Example :

le Maroc = Je vais au Maroc

le Portugal --> Je vais au Portugal

3. For the other case (most of the time, it is feminine name coutry 'la', or name country with 'l' ' before=, we use 'en'

Example :

la France --> Je vais en France

IMPORTANT : a country name starting by a vowel (like 'a, e, o, u') is almost always feminine )
Example :

L'Angleterre --> Je vais en Angleterre

But of course, there is some exeptions

Example :

je vais à Cuba - à Madagascar