Chart showing roman - devanagari character mapping
 
Vowels (dependent and independent):
a      aa/A    i      ee/ii/I     u       oo/uu/U 

e      ai      o      au         aM      aH

R^i    R^I     L^i    L^I
Consonants:
k     kh     g     gh     ~N
ch    chh    j     jh     ~n
T     Th     D     Dh     N
t     th     d     dh     n
p     ph     b     bh     m
y     r      l     v / w
sh    Sh     s     h      L
x / kSh     GY / dny 
R (for marathi half-RA)
(note the diff bet. daryA (sea) and daRyA (valleys))
Specials/Accents:
Anusvara:       .n/M  (dot on top of previous consonant/vowel)
Avagraha:       .a    (`S' like symbol basically to replace a after o)
Ardhachandra:   .c    (for vowel sound as in english words `cat' or `talk')
Chandra-Bindu:  .N    (chandra-bindu on top of previous letter)
Halant:         .h    (to get half-form of the consonant - no vowel - virama)
Visarga:        H     (visarga - looks like a colon character)
Om:             AUM   (Om symbol)
Consonants with a nukta (dot) under them:
k  with a dot:      q (as in qAtil)
kh with a dot:      Kh (as in KhAtir)
g  with a dot:      G (as in GarIb)
j  with a dot:      z / J (as in zamIn)
ph  with a dot:     f (as in afasAnA)
D  with a dot:      .D (as in la.DakA)
Dh with a dot:      .Dh (as in ba.DhanA)