From a beginners point of view this helped me a lot to get going with the SDR stuff. (RTL-SDR v4)

RTL-SDR v4 (Original)
SDR Antenna Set

Boafeng UV-K5 Walkie-Talkie (alternative Firmware / Telegram Group)
Flash Boafeng Firmware via Browser (IMPORTANT: Do a backup of your Radio calibration first!)
Configure via Chirp on PC

Use Airspy for daily scanning your area

Tetra Decoder – DMR Decoder – DPMR Decoder – POCSAG Decoder

ADB-S Decoder with Map view of Airplanes and all Data received

Official Frequency Plan (Germany)

Lookup Frequency (Germany)

Good website about HAM Radio related stuff

Freenet Frequencies (Germany)

BOS Frequencies (Germany)

WebSDR collection

HAM Radio Licence (Germany)

All about SDR, RTL-SDR v4, Airspy, SDRangel, BOS, Tetra Decoder, ADS-B (ADSB), DPMR, DMR Decoder POCSAG Decoder, FM/AM Demodulator, NXDN.