Α (alpha)
Β (beta)
Γ (gamma)
Δ (delta)
Ε (epsilon)
Ζ (zeta)
Η (eta)
Θ (theta)
Ι (iota)
Κ (kappa)
Λ (lambda)
Μ (mu)
Ν (nu)
Ξ (xi)
Ο (omicron)
Π (pi)
Ρ (rho)
Σ (sigma)
Τ (tau)
Υ (upsilon)
Φ (phi)
Χ (chi)
Ψ (psi)
Ω (omega)
Some additional letters used in Greek are:
Ά (alpha with acute accent)
Έ (epsilon with acute accent)
Ή (eta with acute accent)
Ί (iota with acute accent)
Ό (omicron with acute accent)
Ύ (upsilon with acute accent)
Ώ (omega with acute accent)
Also the letters ΓΓ (double gamma) and ΜΠ (double mu) are commonly used in Greek.
To type in Greek, you can use a Greek keyboard or a transliteration tool that converts Latin characters to Greek characters.