That's a really broad question. A coin's date is not the only factor in its value. The denomination, its condition, the mint mark if it has one, and in some cases the design variety within a given year are all factors. The U.S. has issued about 2 dozen denominations of circulating coins over the last 225 years, struck at 8 mints, so you have a huge set of coins to choose from.
If you want to learn about coins I'd suggest going to your local library and asking the reference desk people about a good introductory book. The famous Whitman Red Book is really a retail price guide, but it has enough historical info in it to be worthwhile as a reference book.
BTW, it's not very easy today for someone to go into collecting part time and make any kind of serious profit. The market for a lot of coins has been bid up so much that to make money you have to be more of a buyer and seller than a collector, and do it full-time. Most of us who are collectors do it for the fun of completing a set, learning new things, and being able to display (and brag!) about our accomplishments. With a couple of exceptions, none of us got rich as collectors.
No. The new design coins introduced in 2008, have the date on the "heads" side.
It depends on the coins date and mintmark. Modern DCAM Proof coins are very common and so are Mint State coins.
It depends on the coin. Some are only worth their bullion value, if it's a common date in circulated condition (such as a 1964 Washington quarter). If it's a rare date, or it's in mint condition, then it's more valuable to collectors.
No. They are just more valuable coins. These are the coins from least valuable to most valuable: yellow, red, blue.
1234 coins
Just because a coin was struck at the Denver Mint does not make it more valuable. But some D mint coins are more valuable, it depends on the date, denomination and condition of the coin.
Sure, they're valuable. Money is money, regardless the form: paper note or coins.
DO NOT TRY TO CLEAN VALUABLE COINS. The only coins you should try to clean are common-date coins you get in change. Anything you use to clean a coin may make it look shiny for a while, but the cleaning affects its surface in a way that will accelerate the damage later.
They're not valuable above face value.
No coins in 1949 are rare, but as with all US coins, high grade Mint State examples can be valuable.
Valuable coins can vary depending on factors like rarity, historical significance, and condition. Some valuable coins include the 1933 Double Eagle, 1804 Silver Dollar, and 1913 Liberty Head Nickel. Trusted dealers like Apmex, SD Bullion, and BOLD Precious Metals offer a wide selection of valuable coins for collectors and investors.
Coins have a date to show what year they were made.