Assembly is planned to be added as a language but until then you can use Linux's NASM assembler.
You can learn NASM in @JustAWalrus' tutorial here.
Edit main.asm in the template to start

Example Programs

Hello World

section .data
	msg db 'Hello world!', 0xa
	len equ $-msg

section .text
	global _start
_start:
	mov eax, 4          
	mov ebx, 1            
	mov ecx, msg     
	mov edx, len   
	int 0x80              
	mov eax, 1            
	mov ebx, 0            
	int 0x80

If statement

section .text
	global _start

_start:
  mov eax, 15
  mov ebx, 10
  cmp eax, ebx
  jg greater
  jl less

greater: 
  mov eax, 1
  mov ebx, 0 ; exit with status code 0 if greater
  int 0x80

less:
  mov eax, 1
  mov ebx, 1 ; exit with status code 1, something is wrong
  int 0x80