Am i training my forearms doing scales or am i training my finger muscles? I have very weak fingers, (just started playing 4 months ago.) I started a regime of scales to train up my left, it appears that the pressure i exert sometimes make my fingers feel weak and susceptible to trembling. Is this normal, or am i accumulating injuries?
my forearms feel a bit stiff too. I have very big veins, though doc says it doesn`t hinder my playing, but i`m wary of the dangers of long term playing.

You ought to check your shoulders. Sometimes the forearms feel stiff due to tension from the shoulders.
Scales are technical ex for fingers. U have to understand the purpose and execute them correctly to fully benefit from it.
Get a good hand shape, curve, lift and seperate each finger. Get a good lift, and drop each note naturally with your finger tip...I call that finger `walking`. This will solve your shaky finger problem, though not immediately. Technical development takes time, so be patient.
A good scale must sound `pearly`. That means each note must be crystal clear and round.
If you want to strengthen your left hand, you should progress to doing arpeggios or broken chords. Arpeggios and broken chords work at retaining a good hand shape, and a stronger hand/fingers.
Learn to play with tone shadings for scales when you`re feeling more advanced. Try doing them loud, or soft, or gradual shading.Ie: p cres. to f dim. to p.