The enlisted pay chart starts with basic pay rates for all those who have been in the service for four months or less. That's the amount of time most people spend in basic training, and it's the very lowest rung of the military pay scale. There's a good reason for that--all meals and housing are provided, so you don't get any military allowances.
Pay at the lowest rank for those in training status is considered low compared to those on active duty; just under $1300 per month according to the 2009 enlisted military pay chart. Once you've served more than four months, your pay goes up depending on what rank you were given when you enter basic training. Most enter military service with a rank of E1; those who sign up for six-year enlistments or enter with other considerations may be E-2s or even E-3s in some cases.
From there, your military pay goes up depending on two factors:
The pay increases for your time spent in a specific rank begin when you hit E-3. There is a pay increase starting when E-3s and above spend more than two years in that rank, more than three years, and so on. You don't get an automatic yearly increase in pay; at some ranks you may have to wait two years. That's one of the reasons why the enlisted pay chart can be so confusing. In some parts of your career your military pay goes up yearly, at others it does not.
MILITARY ALLOWANCE INCREASES
Your military allowances increase with your rank, but not your time in service. When you get promoted, you're entitled to more money from your allowances as spelled out in Finance regulations. Each allowance has its own nuances, but these amounts only go up when you get promoted or when the government announces an increase in the overall allowances per rank.
There is other money you may receive from the military; re-enlistment bonuses, travel pay and tuition re-imbursement. These are all very situational and much depends on current regulations which govern these types of compensation. When it comes to your basic pay and allowances, some terms or amounts may change from year to year, but the core system remains the same.