Spanish spelling (ortografia, grafia) is pretty consistent: most letters represent a single sound regardless of their position in a word.
Note the following peculiarities:
H - la hache
Thus, words like Honduras, ahora and alcohol have no aspiration before the /o/ sound.
La hache is not combined with any other consonants.
There is no th, sh, ph, gh, etc.
English "ph" may translate to "f": filosofía, Filadelfia, fantasma.
CH - la che
Is always pronounced as in "cheers": coche, ocho.
C - la ce
Is pronounced /k/ (as in "case") in most positions: caso, cosa, cuota, frecuente, crisis
Before -e, -i, it is pronounced /s/ (as in "sin") in America or /th/ (as in "thin") in Spain: cielo, acento
The /k/ sound (as in "kiss"), is spelled "qu" (mute "u") before -e,-i: queso, quince.
G - la ge
Is pronounced /g/ (as in "go") in most positions: gala, gota, guante, globo.
Before -e, -i, it is pronounced almost like /h/ (as in "hen"): general, gitano.
The /g/ sound (as in "get"), is spelled "gu" (mute "u") before -e,-i: guerra, guitarra.
If the letter "u" is to be pronounced in a "gue/gui" combination, it is marked with a diaeresis (la diéresis): pingüino, bilingüe, nicaragüense.
Q - la ku
Is used only in the que/qui combinations, and the "u" is always mute in this position.
Therefore, the word quinteto has no /u/ sound, and English "quota" and "frequent" translate to cuota and frecuente.
Z - la zeta
Is pronounced /s/ in America and /th/ in Spain.
Spanish avoids the ze/zi combination and prefers ce, ci: lápiz >> lápices; cebra, cenit.
Only four consonants can be duplicated to represent specific sounds:
cc is used before "e" or "i" only and sounds /ks/ (/kth/ in Spain): acción and acceso but acento, ocurrir.
ll sounds /y/: calle, llama but ilegal, aludir, inteligente.
rr represents the famous "rolling r" between vowels only: perro, carro, vs. pero, caro.
nn is used only when a prefix ending in "n" is added to a word beginning with "n": innecesario, connotación, but anual, anotación, conexión.
No other consonants are duplicated in Spanish: efectivo, común, oportunidad, imposible, adición.
Summary / Resumen
Never pronounce the letter H (hache): alcohol, ahora, humano, Honduras, holocausto.
Hard C sound: /k/ as in kiss
ca que qui co cu
S/Z sound:/s/ or /th/ as in sink/think
za ce ci zo zu
Hard G sound: /g/ as in get
ga gue gui go gu
Soft G sound: /h/ as in hen
ja je ji jo ju ge gi
As you see, vowels e and i are exceptional in their combination with g and c.
The words que, quien, guerra and guitarra may help you remember these spelling changes.
No double consonants except rr, ll, cc and nn.
ph --> f : filosofía