There are several ways to level a lawn, all of which require much work, time, and energy.
The first way is to till that section and level it and then reseed or sod the area. Probably the best way but certaainly the one requiring the most work as well as the most time.
The second would be to bring in some soil and carefully rake it over what is there to even things out. This is not always the most effective method and, depending on the depth of some of the depressions, could kill the grass that is there now.
A third method is to make X's in both the high points and low points and remove some soil from the high points and use that to fill in the low points. A lot of work and much less expensive then the others since the only cost is your labor.

It is very seldom when I disagree with kimmsr, but this is one of those times. The leveling topic has been addressed at length in the other lawn forum. Over the years of reading the successes and failures of others, I have come to understand that tilling is not the right way to prepare the soil for a lawn. While it might seem to work for a veggie garden, when you till for a lawn you will almost always end up with a lumpy bumpy lawn after all the soil settles (3 years). Why? Read on...

A hand held rototiller is a beast to drive. Imagine trying to steer a bucking bronco. It is impossible to hold the thing steady as it chews through soil that is soft, hard, moist, dry, cloddy, fluffy, rocky, rooty, and otherwise non uniform. What you end up with is a lot of fluffy soil on an uneven base below the fluff. When you level out the surface you end up with a layer of fluffy soil of different depths before it reaches the hard, untilled, soil underneath. When that surface fluff settles in 3 years, it will settle unevenly. That might be what you have now. Someone might have rototilled to plant tomatoes a few years ago and then let the veggie garden go back to grass.

Check out the regular lawns forum for more on leveling. In a nutshell it goes like this...

1. Wait until your grass is growing like wildfire. For bermuda the time is July. For other grasses the time is more like April.

2. Feed and water well right before doing this. I like organic fertilizer because you can overfeed without hurting anything.

3. Bring the grass height down carefully before you finally scalp it.

4. Spread bags of sand with 2x4s, drags, and whatever else you can find to make it level. Brooms will not work because they are not wide enough. Brooms help to move sand around, but you need something big to really level it.

5. Water to settle the sand and add more sand where it looks less than your desired degree of level. Water the new sand and recheck. Sand, level, and water until you are happy.

6. When you are finished with the sand, start watering like it was new grass. That means keep it moist all the time. 3x per day for about 10 minutes each time works.

Unfortunately this watering regimen in April will result in a crabgrass lawn in July. With that in mind, there are some people experimenting with scalping and leveling in the winter while the grass is dormant or otherwise not growing.

So in essence, I disagree with kimmsr's first suggestion. I do agree with his second and third suggestions. What I provided above is a lot of detail on his second method. Read the other posts on the leveling topics for a lot more help.