Answer:
Goodbye: there are several ways of saying this.
"Da bo ti" (familiar) and "Da bo chi" (formal) are the usual ones.
There is also "ffarwel", which to be honest I have only ever seen in print. If you want to be casual, then you can say "hwyl". which translates as "cheers".
Hello: "helo" or "shw'mae?". The latter is a way of asking how you're doing which doesn't expect any reply.