Where are you getting the shrimp? With only 6 shrimp there is no guarantee you'll get a male and a female. There are numerous posts on here from people who bought 10 shrimp and who figured out that they wound up with 10 females a few months later when the shrimp wouldn't breed.
If it's via mail order, politely ask the sender to try to include both males and females. There's still a reasonable possibility of getting a single sex batch, especially if you're getting juvenile shrimp.
If it's from a store, look for adult shrimp, well colored. Females are slightly larger, have thicker tails, and tend to much redder. They also often have either eggs (are 'berried') or their ovaries are prominent (they are 'saddled', visible as a large yellow patch across the back.) Males have thinner tails, are slightly smaller, and tend to be more poorly colored (and of course they won't have either eggs or ovaries showing...) It's fairly easy to ID the females, but the males and juvenile females can look an awful lot alike.
If you're ordering from a hobbyist, locally or via mail, you're in luck. Again, ask for both sexes and request at least 1 adult of each. Anyone who keeps shrimp for long should be able to ID the sexes fairly well.
Once you get the shrimp and they've adjusted to your tank (which I assume is cycled and free of predators, usually a betta in a tank like this) then they'll breed. It might take them 3 months to start, and if they're juveniles when you get them it'll be another 1-2 months on top of that, but once they start they don't stop.