Here are a couple examples using SNMP v2c and v3.
SNMP V3 with authentication
snmpwalk -v3 -l authPriv -u USERNAME -a MD5 -A "PASSPHRASE" -x AES -X "PASSPHRASE" device.domain.com 1.3.6.1.2.1.31.1.1.1.1
SNMP v2c
snmpwalk -v2c device.domain.com -c SNMPString
Sometimes you need to redirect this to a file for better searching and reviewing.
snmpwalk -v2c device.domain.com -c SNMPString > mibtree.txt 2>&1
That will do it without any errors or unwanted output.