A bag of squid can be purchased at a bait shop next to the marina in Rocky Point. We've even used pieces of hot dog to catch trigger fish and spotted bay (rock) bass. Small, hungry fish will attack about anything.
As for where to fish, you should ask at the Mayan Palace as they've undoubtedly heard this question before. But I'll give you one tip that might or might not pay off.
From the Mayan Palace, drive out to the highway and turn left (I think, or is it right?) and drive a couple of miles to near milepost 32 (I think) where you'll see a road going west at the ruins of an old house. Take that road a mile or so toward the ocean and you'll find yourself at La Pinta estuary. There will be some boat trailers and a couple of men there keeping an eye on them. Nothing to worry about. You're welcome to fish there. You can also dig for clams farther down the beach to the south.
Try Kastmaster or Crocodile lures for corvina (sea trout). Used to be you could catch them at La Pinta and elsewhere around Rocky Point, especially around sunrise and sunset, but that was before corvina became nearly extinct from net fishermen and shrimp boat by-catch. I haven't fished around Rocky Point for about five years as I've been going farther south to Puerto Lobos. We haven't seen corvina down there for at least three years, although I've heard some have been caught around Santo Tomas. I'd appreciate hearing a report from you.
Others with more recent experience at La Pinta might wish to weigh in.