Doubling. The first thing I notice is that the lead vocal is doubled at the octave. It is also harmonized, possibly by using a vocoder. This is what gives the line its harmonic richness.
Chorus. The second thing I notice is that the vocal line is doubled in stereo space using a chorus effect. In FL studio you will want to use a stereo track for the vocal line and process it through a decent stereo chorus. This is what gives it the spatial richness.
Echo. The third thing I notice is that the vocal line has a soft "echo" (delay with feedback). It's hard to notice because the attack of the echoes is very gentle. You can get this effect by running the vocal through a reverb with a longish predelay and a shortish decay and then through a delay. The delay in this case is proportional to the tempo, so you'll want to use FL Delay (and not FL Delay 2) and set Tempo to "Auto" and Steps to an even number. This is what gives it the temporal richness. Experiment with either "Inv Stereo" or "Ping pong" modes.
Reverb. At the end of the chain you'll want to run everything through another reverb (yes you can have two!), this time with a longer decay and large room size (perhaps using a variant of the cathedral preset), to make it sound spacious.
EQ. To avoid making a huge harmonic mess, you'll want to sort of "interleave" the lead vocal and the backing vocals by using opposite parametric equalization. The background vocals seem to have a lot of high pass while the lead rolls off some of the highs and puts emphasis around the meat of the male voice, around 1K-3K hz. So you'd decrease the mids on the backing vocals to let the lead voice come through more clearly. The equalization should be applied before any of the other effects in the signal chain. Well, almost any... which brings me to compression.
Compression. To get your vocals to sit well in the mix, it is very helpful to compress them. It's counterintuitive-- we tend to think of compression as making things small-- but compression doesn't actually decrease the peaks, it raises the valleys. This evens out the dynamics and make the parts seem less wimpy. Compression should be applied to each channel individually, right at the beginning, before EQ.
Master limiting. Then at the end, put your entire mix through a hard knee compression. This is known as master limiting. Use Fruity Limiter on the main output and raise the gain to taste. This will make the overall track thick and consistent.
