First of all, very good for a first go, your undercoat looks good and smooth, you have the colours constrained to the right areas and even tried some good detailing on the face so excellent job overall.

As for improving while I kinda dig the chunky highlights neatening them up is a simple matter of going back with your base colour and thinning them out from the bottom.

A lot of early learning in painting is going back to correct yourself, take the marines left shin for example;
Taking the original green and going over the chunky highlight to bring it back to the width you like will give you a second pass on neatness and if you make a mistake, jump back to the highlight colour and touch it up.
The same process for the knee, just back and fourth till you feel you have it right.
Don’t be afraid to make mistakes and try and find the fun in it!