... the manual helps ... it is something like
DSolve[{D[S[t], t] == \[Alpha] - \[Beta] S[t] H[t] - \[Mu] S[t],
D[H[t], t] == \[Beta] S[t] H[t] - (\[Gamma] + \[Lambda] + \[Mu]) H[t],
D[T[t], t] == \[Lambda] H[t] - \[Mu] T[t],
D[A[t], t] == \[Gamma] H[t] - (\[Delta] + \[Mu]) A[t](* initial conditions *)}, {S[t], H[t], T[t], A[t]}, t]
if you must go to numerical solutions (NDSolve[]), the coefficients must be specified ... the I is reserved for the imaginary unit in Mma, so use e.g. H instead of I
