It is my first song , and I don't know what to do...
This is a basic way to make ReBirth sound good :
First of all , load a new window . Turn off  rhythm section and one 303 . In the upper 303 , turn delay to the right , and play  the pattern . Now make  Edit/Randomize accents , etc (Ctrl+R) ...and MAGIC! you probably have now a cool bassline . Turn CutOff  and/or Resonance . If you don´t like the pattern , just Ctrl+R again .You can change the base pitch with  transpose , or modify it slightly changing a couple of notes .
The second step is the  rhythm section . The most basic  rhythm would be this : select BD (bassdrum) and place 4 notes , one at the beggining of each colored section (red,orange,yellow,white) . Now select CH (closed hihat) and place another 4 notes , but delayed two steps from the BD notes . Veterans from the days of  DOS Trackers , will find this familiar .
Now comes the difficult task : creating a pattern for the other 303 in harmony with the first . And of course , more than one is needed . Playing a piano keyboard or a "virtual piano" in Cakewalk , hearing at the same time the other patterns  will help . When you have something acceptable , translate the pitches to the 303 , and play with accent/decay , etc. You will notice that the same combination of piano pitches may change to infinite variations in the 303 , some odd , and some great .
ReBirth offers you a lot of nice controllers to make new sounds , such as distortion , feedback, pan ...
And if you don't get any harmonic sound , it doesn't matter . You always can proclame yourself as the creator  of a new style such as "HardTrashAcidTechno-Trip" or similar . Who knows...
 

Random is not *always* = Chaos
How did you thought most 303 songs were created ?. Could anyone imagine beforehand what exact sound will produce a pattern  with infinite combinations of knob settings , accents , slides , etc? And ReBirth randomize function produces a high percentage of really harmonic patterns .
But you cannot make a song totally at random . If it were possible , ReBirth would  not need YOU .

Calculate your song's lenght

This is a useful formula :
song length in minutes = ( number of bars x 4 ) / Tempo

Think before you start recording , that a 5 minutes song with a tempo of 140 bpm requires ¡175 bars ! ...Well , it seems that you will need more than that couple of good patterns to fill the song . Or maybe not...

ReBirth is not only cool for music
You can create great sound effects too : Try with patterns with one or two notes only , and decay and delay to the right . Then play with resonance , modulation and cutoff .

808 is not enough
Surely , one of  improvements in future versions , will be the 909 section . Till ReBirth 2.0 comes , you may use a real 909 , or emulate it with two great progams :
Stomper Ultra , wich is a drum sounds generator
Hammerhead , a complete sequencer , 909 styled
Then , adjust the tempo , and mix it all with a tracker or MIDI  sequencer . I recommend Cakewalk Pro Audio 6.0

Compare my "ReBirth RB-338" logo.....with the original from Propellerheads

 

Take which  you prefer ...even if you are from Propellerheads .