Nationals of the European Union do not need a VISA to enter Belgium and are allowed to stay for as long as they like. They are however, required to do some paperwork, especially if you stay in the country for longer than 90 days.
For further information check: https://dofi.ibz.be/sites/dvzoe/EN/Pages/home.aspx
Belgium is signatory of the Schengen agreement (signed in Schengen, Luxembourg), which allows free movement within the EU member countries, plus Iceland, Norway, Switzerland, Monaco, San Marino and Vatican City. Each member nation retains the right to refuse to allow you to enter, even with a valid visa. However, unless you’re obviously carrying drugs or smuggling in refugees or something illegal, you probably won’t even have to slow down as you cross the various Schengen borders.
For people who live outside of the EU you will need to apply for either a short-stay or long-stay VISA. Do not attempt the country without one as you will be refused entry. Belgium requires foreign nationals to provide evidence that you have a work permit or evidence that your employer has applied for one on your behalf.
To apply for a long-stay visa, you’ll need a passport that’s valid for at least 12 months plus some or all of the following documents:
A legalised birth certificate (i.e. with apostille) and translation for each member of your family.
Legalised (apostille) copies of marriage or divorce certificates with translations, as applicable.
A medical certificate from an approved doctor (the consulate or embassy will give you a list).
The completed application form(s).
Passport-type photos (one to six or more, depending on the circumstances).
A work permit (if you’re planning to work) or evidence that your employer has applied for a work permit for you.
Evidence of eligibility to practise your trade in the country (if you’re self-employed, a journalist or in some other regulated profession) or a professional card, which serves as a work permit for some self-employed trades.
A police report (sometimes called a ‘certificate of good conduct’).
Proof of financial resources (if you aren’t going to take up employment).
Proof of health insurance.
If a student, evidence of admission from an approved educational establishment.
If an au pair, a copy of your agreement with a host family.
If you want to become a citizen of Belgium you have to be over 18 years of age, have lived in the country for the last three years and must have genuine attachments to the country, such as a job, friends and family, investments and debts. You then need to fill out the relevant paperwork and provide some documentation, including copies of your birth certificate, proof of Belgian residency and any other supporting documents.