I need to format a partition.
But I have one LVM on my machine (VirtualBox) that is composed of two different two partitions of two Virtual HDD's (sdb5 and sdc5)
fdisk output 
df output

I need to format a partition.
But I have one LVM on my machine (VirtualBox) that is composed of two different two partitions of two Virtual HDD's (sdb5 and sdc5)
fdisk output 
df output

LVM doesn't change the way you format a partition. Let's say you would have a volume group called group1 and a logical volume called volume1 then your command should look like this for ext3:
mkfs.ext3 /dev/group1/volume1
In case you don't have any volume groups or logical volumes yet, you have to use the according LVM tools to create them. The manpages of vgcreate and lvcreate can tell you how to do that.