'pesan' (noun) is the base word.
By using the 'me-' prefix, the base word is converted into a verb.

[base word] + [me-] = (verb)
'pesan' ==> 'memesan'.

When converting a base word into a 'me-' verb, the new word sometimes need to be 'conjugated'.
It depends on the starting letter of the base word if "me-" needs to be 'conjugated'.
The conjugation table looks like this :

Starting letter base word	Conjugation
l, m, n, r, w	Me + base
vowel, g, h	Me + ng
c, d, j	Me + n
b, f, v	Me + m
k	Me + ng – k
t	Me + n – t
s	Me + ny – s
p	Me + m – p
monosyllabic	Me + nge +base

For example :
lihat ==> melihat
ajar ==> mengajar
cuci ==> mencuci
baca ==> membaca
kejar ==> mengejar
tarik ==> menarik
sapu ==> menyapu
pilih ==> memilih
cat ==> mengecat