how do u play low c# and lower

If you have gotten down to C I almost assume you have a finger chart. Which I would assume would go down to A.
But, if not then here are the fingerings:

C# 123 C#/ 123C
C 123 /123C
B 123B /123C
Bb 123Bb / 123C
A 123A / 123C

If you do not know your pinkie keys here is the legend.
LEFT HAND:
The top pinkie is G#
The bottom pinkie is Bb
The left pinkie is C#
The right (or inside pinkie is B)

RIGHT HAND:
The top pinkie is Eb
The bottom pinkie is C

Now, if you already knew these fingers and your instrument is not playing the notes, then we have other issues. If this is a Selmer then put your mouthpiece/reed guard in the bell and see if that helps (yes, just drop it in the bell). Or you could have leaking at one or more pads.