Assuming the account running "JobA" has the authority to start jobB, the command in JobA would be:
EXEC msdb.dbo.sp_start_job 'JobB'
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.