The usual minimums are as follows:
Inside bets--on individual numbers, or groups of numbers (called splits or corners) -- $5 total. You can make five $1 bets if you like, but they must be on the inside part of the board where the numbers are.
Outside bets--on odd/even, red/black, dozens, columns, 1-18/19-36) -- $5 per bet. You must play at least $5 on each and any of these bets, with no splitting.