The different-sized and shaped coils are for specific purposes. Larger coils allow you to cover more ground with each sweep. Smaller coils are for hunting in confined spaces, like rock crevices.
The cheapest one I had was a Bounty Hunter. It has a display and comes with a medium-sized coil. You can get an optional coil for it for nugget hunting that is round and only about 4 inches in diameter. That concentrates the signal, making it more sensitive.
Machines come in all sizes and weights. If you are serious about trying it, see if there is a metal detector dealer near you. Talk to them about what you want to do: coin shooting, beach hunting, gold nugget prospecting, etc. They can recommend a machine for that purpose.
Many will let you rent a machine for a weekend so you can try it out. If you get one that is too heavy, it will wear you out after a couple of hours. My dad had several back surgeries, and now he can't use his Minelab because of the weight.
The Bounty Hunter 505 is a good machine. I used mine for more than ten years before I upgraded. I had to, because I moved out of state and forgot it in my garage. The Bounty Hunter is great for starting out. It will easily find coins and jewelry, is lightweight, and is cheap. I got mine at Walmart ages ago for $75. I later bought my wife an upgraded version for $150.
The most important thing to remember is that it is a machine. It cannot lie to you. If it beeps, there is a reason. It is up to you to learn what the different beeps and tones mean. That comes with practice.
I had a practice garden in my backyard. I swept the ground and removed all metal from it. Everything. Then I went back and put in various coins: a penny, a dime, a nickel, a quarter, and a half-dollar. I then added a couple of nails of different sizes, a bottle cap, a pull tab, a fishing sinker, and a piece of necklace chain.
Then I would practice by learning what each target sounds like and what the display shows. Some brands include a DVD or video to demonstrate how to use it and practice with it. Over time, you get to where you don't need to look; you know instantly, "Oh, that's a nickel."
Then I moved into Advanced Training. Instead of having every coin lie flat on the ground, I placed them at different depths in the ground and at various angles. Some completely on edge. Sometimes I would lay a nail across a coin to confuse the signal to see if I could pick out what was there.
When I was confident enough, I would go to local parks or roadside picnic areas. Again, let the machine tell you what it is before you dig it up. You will find a lot of trash. Bag it up and take it with you. The valuables pay for your cleanup effort.
I have also done well on beaches, especially swimming holes, campgrounds, schools, and places with parking lots. My dad knows a woman who did well in Walmart parking lots, checking the landscaping stones. Always check local rules and regulations, and if you are on private property (like Walmart), get permission first.
