Roni, you will need to use 100% silicone caulk ("kitchen, bath, plumbing" or "tub and tile"). Unfortunately, you really should remove all old caulk as silicone doesn't tend to stick to cured caulk very well. I had to do this in the corners of our PC shower. Take a plastic putty knife to scrape old off. If this doesn't work, you could CAREFULLY use a metal scraper (like a 3 in one tool). I say carefully, because you don't want to scratch the shower surface. I even had to use a silicone caulk remover to get all the old residue off. Remember.....it's all in the prep!! Good luck. Not a fun job.
