Your coin was used in Germany up till the country switched to the euro in 2002. If its value is in pfennige or is at most 5 marks, it's most likely to be an ordinary circulation coin.
If so, it's no longer spendable but its value at the time of conversion was roughly half as many US dollars or cents as its denomination in marks or pfennige; e.g. a 10 pf coin was worth about 5 cents.
If not, please post a new question with more details including its denomination.
Your coin was used in Germany up till the country switched to the euro in 2002. If its value is in pfennige or is at most 5 marks, it's most likely to be an ordinary circulation coin.
If so, it's no longer spendable but its value at the time of conversion was roughly half as many US dollars or cents as its denomination in marks or pfennige; e.g. a 10 pf coin was worth about 5 cents.
If not, please post a new and separate question with more details including its denomination.
How is my old coins worth
We need more information. The words "bundesrepublik Deutschland" just mean "Federal Republic of Germany" and are not the actual value of the coin. The coin will have a number on the front, representing its value in Marks (or Pfefning).
There's no such coin. Germany did not take the name Bundesrepublik Deutschland (German Federal Republic) until 1950.
It is a German Deutsche Mark.
Money
Please post a new, separate question with the coin's denomination.
There is a website called Numista that has coin pictures. Also on eBay and so on ...
This coin is no longer in circulation, having been replaced by euro-cent coins in 2002. At the exchange rate in effect at the time, it was worth about a nickel.
About $2.50 in normal circulated condition, up to maybe $40 if almost new-looking
This was a coin produced for mass circulation. Even to a collector it would have very little value. What would, however, increase its value would be if it were a mis-struck coin, e.g. off-centre, numerals or letters missing. Check out www.muenze.com for more information.
5DM was the only German monetary unit that was issued as both a note and a coin. Your question does not say which you are asking about. It was standard circulation, so unless it is in "brilliant uncirculated" condition or has a flaw it will have little intrinsic value.
It depends on denomination, mint mark and condition - as little as a few cents and as much as $15 or so.
The 50 pf denomination was moderately common in Germany until 2002 when the euro was adopted. sammler.com, a major German collectibles site, lists a value of about US$2.75 for an uncirculated coin. In circulated condition it's only worth face value, about 25¢ when Germany switched to the euro.