is welsh spelling always phonetic? so far it's been spot on. although y and w make like three sounds each.
Pretty much so, yes.
If you go to the list of discussions, click on 'Popular' and then the third one or so down is to do with 'Pronunciation' - in the initial post there, there is a link to a series of Youtube videos on pronunciation. Bookmark those and go over them once a week or so - they are about the best guide around to basic Welsh pronunciation.
w has one basic sound, although it can be long or short, as with other vowels. y, unusually, has two basic sounds - the videos explain it all. The videos give two alternative regional pronunciations, but there are several others. Perhaps stick with what they call 'southern' unless you are going to be using your Welsh regularly in NW Wales - it is slightly simpler to begin with and it is fairly close to how Welsh is pronounced in mid and west Wales, too.
eisiau is a common exception to the phonetic guide - it is usually pronounced /isha/, /ishe/ or /isho/. sut mae? is another - /shwmae?/, /s'mae?/ are common variants.