What cassete do I need to run with a compact crankset (50/36), to equal the same gearing as running a standard crankset (53/39) with a 11-28 cassete? Every new bike in my price range comes with a compact crankset, which I want for climbing, but I race and I need to keep the same top speed that I've had for sprinting and descending (I live in the mountains). So I'm just wondering what rear cassette I would need to use with my compact cranks to equal the same gearing as using a normal crankset (53/39) with a 11-28 cassete.
It's not possible. You'd gain lower gears, but lose your very highest gear. If you want to keep that highest gear, maybe you should look at a wider-range cassette to get there, instead?

If you were switching from a 53/39,12-28 setup to a 50/34,11-28 setup, it would work out well.

Switching your largest chainring from 53 to 50 basically shifts all your high gears about 1 notch lower.

11 teeth is the smallest cog you can get on a road bike cassette, currently. I've seen BMX bikes with a smaller gear on the back, but they're not cassette based.

You can compare gear ratios on a given bike (where wheel diameter, tire width and crank length can all be assumed to be a constant with simple division.

Assuming your 11-28 is 11-12-13-14-15-17-19-21-24-28, or similar:

53:11 = 4.8
53:12 = 4.4
53:13 = 4.1
...
39:21 = 1.9
39:24 = 1.6
39:28 = 1.4   

50:11 = 4.5
50:12 = 4.2
50:13 = 3.8
...
36:21 = 1.7
36:24 = 1.5
36:28 = 1.3
... or if it's the more common 50/34:
34:21 = 1.6
34:24 = 1.4
34:28 = 1.2