Education delivered through the medium of Welsh language is known as Welsh medium education. Bilingualism is important for children in Wales - throughout their lives, a child's choices and opportunities will be affected by whether they are mono-lingual (speak one language), bilingual or multi-lingual.
Welsh became a compulsory subject for all pupils in Wales at Key Stages 1, 2 and 3 (i.e. up to age 14) in 1990. In 1999, it became a compulsory subject at Key Stage 4. So now all pupils in mainstream schools in Wales study Welsh (either as a first or a second language) for 12 years, from the ages of 5 to 16. Provision in independent (private) schools is less widespread - a few teach Welsh as a second language but there is no designated Welsh medium private school in Wales (though there is one in London).
So, unless they go to one of the few private schools in Wales, your child will either be taught mainly in Welsh, or learn Welsh as a second language. Depending on where you live in Wales, you may well be able to choose whether your child attends a school which predominantly teaches in Welsh or in English.
There's lots to think about - so here's some information to help you:
On this site, you'll find also a list of organisations and useful addresses, reading and websites with more information. 
