Error on sending AT+CWJAP_DEF commands to ESP8266
Based on the gibberish echo, the possible causes are:


Insufficient power. You need a source that can supply up to 300mA which is about the max consumption for the module. A good-sized capacitor across the ESP power rails (10uF maybe) wouldn't hurt.
Baud rate is too high. If you're using an Arduino as 'middle-man' and talking to the ESP8266 through a SoftwareSerial port, you're likely to face this sort of issue if the baud rate is as high as 115200; SoftwareSerial is unreliable at such rates.