Let C1 be the charged capacitor and C2 be the uncharged capacitor.
Initial charge Q0 on C1 is: Q0=C1V0 being V0 the initial voltage across C1.
Initial energy stored in C1=(1/2)Q0V0
When you connect both capacitors in parallel C1 transfers some of its charge to C2.
By the conservation of charge principle the final charge Q1 on C1 + final charge Q2 on C2 = Initial charge Q0 on C1 and the voltage V across both capacitors is the same (they are in parallel).
So you have:
Q1 + Q2 = Q0
Q1 = C1V
Q2 = C2V
which you can solve for V and then obtain the values of Q1 and Q2
The final energy of the two capacitors system = (1/2)Q1V+ (1/2)Q2V which you will find is less than the initial energy stored at C1due to heat and electromagnetic losses.