In the "good old days", you could easily spot their bright yellow color, but today most Telekom booths have been changed to a "stylish" pink-lavender which is harder to spot.
To call you will probably need a telephone card. Some phone booths still accept coins, but these are very rare, so don't depend on it.
Telekom cards can be bought at the T-Punkte of Deutsche Telekom, post offices, and many stationery shops in denominations of €5, 10, and 15. You can also buy prepaid cards from other providers which can save money and are used by dialing a PIN. Credit cards can be used from a small number of public phones.
In most public telephone boxes it is also possible to be called back, if so the number will be displayed.