Ok the first and most important thing to realise is that dB is a relative scale, it is essentially a ratio but because we deal with such large ratios it is also a logarithmic scale to make the numbers more manageable.
So to use dB as a measurement we need to specify a reference point. For dBu, the reference - 0dBu is 0.775volt. Now dBu is a measurement of voltage or signal level. For dBfs, the fs stands for full scale and is only relevant in digital settings and so 0dBfs is when all the bits are a 1, ie it can't get any higher.
To relate the dBu and dBfs scales you need to establish what your headroom figure is going to be and like all good standards we have more than one. Some manufacturers set -20dBfs as 0dBu, the standard here in the Uk is -18dBfs is 0dBu. So you have to know what your metering has been set to, to be able to equate dBu ad dBfs.
Hope that helps,