Welcome to HCM.
How frequently do you masturbate?.
If you are a regular mastubator, then the semen may become thin and less in quantity.
Reducing the frequency should help.
You will need to get your semen analysis
done and if the sperm count
is low, you will need a Colour Doppler Study to exclude varicocele
If frequency is not the cause for thinning, then best is to consult an Urologist
who will have to examine you and do investigations to find out the cause and treat accordingly.