Unfortunately there isn't an eye datum point built into most lighties. But as a guide you should be able to see the cowling just behind the propellor. Hmm...jargon - the 'cowling' would be the 'bonnet' if the aircraft was a car. Use cushions if necessary.
Your seat should also be just far enough forward for you to press each pedal on the floor (the 'rudder pedals') to the limit of its travel. You shouldn't have to stretch to do it. For a most aircraft, including the C150/152, that also include the extra set of pedals (the brake pedals) attached to, and above, the rudder pedals.
Re. hard to control etc. One of the best things you can do is hold the control column in your finger tips - a bit like holding a pencil so it can hang from between your thumb & 1st finger. Don't try to 'squeeze' the control column though, more like a 'push' using your thumb or a 'pull' using your finger. If more force is necessary then use a couple of more fingers to pull or the heel of your hand to push. That would normally be transient though so you would get back to the lightest grip that you can as soon as you can.
Don't try to look at the instruments or the controls while trying to move them. Look at the Earth's horizon directly in front of you. Your goal is to move the nose of the aircraft in relation to the horizon using a 'pressure' on the control column, rather than to move the control column as such.
The C150/152 has an armrest built into the door. I recommend you use it to support your arm while you hold onto the control column. That way your arm will be more relaxed from not having to support itself and you will tend not to have as tense a grip on the column.
Hope this helps. Good luck with your flying.
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Don't mess with the red switches.