You sound like one of the few cases where I think medium load higher rep work would actually be beneficial. First it will probably help to add a little mass on your pretty slim frame but as you are almost a complete novice the higher reps will help to build the soft tissue strength necessary for heavier lifting. I'd stick to multi-joint exercises; do 3-6 exercises per session; 5+ sets of 8-10 reps for your priority lifts; and 2-3 sets of 10-12 reps for your auxilary exercises. After one cycle (~4 weeks) I'd suggest switching up the exercises and the set rep schemes so that you're handling slightly heavier loads for a lower number of reps.
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