As Tess said, commission-only jobs don't guarantee a monthly amount. You may, however, want to ask for a draw against commissions. In this case, the employer will give you a set amount each week (say $400). At the end of the month (assuming commissions are calculated on a monthly basis), the $1,600 you have received (4 x $400) is deducted from commissions you actually earned and the rest is paid to you. So if you earned $2,100 in commissions, you would get an additional check for $500 ($2,100 - $1,600).

The draw can be "recoverable" which means if you earn LESS than the $1,600 they advanced to you, you have to make it up the next month. In that case let's say you earned $900 in commissions in the month. You would own them $700 ($900 earned - $1,600 advanced) so the next month you would have to earn $2,300 in commissions to break even ($1,600 for that month  + $700 from the previous month).

If the draw is "nonrecoverable", if you get in the hole one month, they can't take it back from you the following month. But if you do this a couple of months in a row, your job will be on the line.

IF you have a solid sales record you can argue that you will easily top the draw and there will be less hesitation from the employer to give you one. But if you are new at this (and it sounds like you are), the employer will be less likely to accommodate you until you prove yourself.