How to calculate the Impedance of a circuit
I always get a description of what to do if they are connected in series or in parallell

That's ultimately all there is.  You keep applying parallel and series combinations until there is only a single impedance left.  For example, in your first circuit, R and L are obviously in series.  Solve for that.  Now compute that result in parallel with C.  In the second circuit, you can likewise combine R2 and C1 in series, then the result of that parallel with R1, then the result of that in series with L.

The "//" operator means in parallel with.  The arbitrary case of multiple impedances in parallel is the reciprocal of the sum of the reciprocals.  R1 // R2 // ... Rn = 1 / (1/R1 + 1/R2 + ... 1/Rn).

Note that for just two impedances, this can be reduced to (R1 * R2)/(R1 + R2).  Your second equation for parallel impedances is wrong because you forgot to take the reciprocal of the resulting sum.

Note that the math above works not just for resistances, but also arbitrary impedances.  In the general case, these are complex numbers.  If all the impedances are resistances, then all the values are real numbers, making the computation easier.  However, the algebra is the same, except that it has to be carried out on complex numbers in the case of arbitrary impedances.