Don't worry about bulking up or cutting down at this stage. Just worry about getting sufficent calories to fuel yourself. If your excessively sore or tired the next day then look its time to look at your diet. Make sure your getting in 5 meals at least 500 cals each. Go for around 3000+ each day at least.
Personally, I would start doing full body workouts and make sure I feel all my movements. Are your lats contracting hard on pull-ups or lat pulldowns? Are you squeezing your chest on bench press? I would do as many compound movements as I could handle considering your core and what type of load that could handle without going overboard. Pushups, pullups, dips, etc. are a good place to start.
You remind me of myself, I'm 6'0 and 148 pounds. Used to be 230 then over a couple years dropped to 140 . I think i'm too skinny but that'll change, better then being too fat I guess.