Welcome to TNet. I see you have 25 posts. I'm guessing you haven't had a detector before? If no, don't hurry out to buy a detector ... yet.
Search for Metal Detector FAQs. Read them. This should answer many of your initial questions, but will suggest more questions. Write them down.
Now you are ready to contact a metal detector store and salesperson near you. Ask your written-down questions of the salesperson. Allow yourself an hour or more. Most salespeople are going to want to sell you a detector. You want a salesperson who will help you find treasure.
Finally, a detector is only as good as the person using it. Most good detector stores will have an area where you can test a detector. Take the test drive. Find out what detector is easiest for you to use, and fits your needs. You are the best and ONLY source for answering this question. If a salesman tries to force you to buy today, DONT! Put that decision off until you can see another salesperson. You owe it to yourself to become knowledgeable.
I'd suggest (if you haven't already) buying a treasure magazine or two (or more). Used ones are fine. Lost Treasure and Western & Eastern Treasures are good. Read them cover to cover, including the ads. Learn the code words and what they mean. Read about what is being found and where.
Finally, you probably won't make a lot of money detecting. At least not at first. Like any hobby, it takes time. If you don't want to invest hundreds of hours learning your detector and digging up lots of junk, don't buy a detector! There are few quick finds. As Indiana Jones said "... X never, ever, marks the spot." All a detector can do is tell you where to dig: you have to put the detector over the spot first.
Hope to hear more from you!