Over all Advise please

I just started working out, I use to work out some but it never lasted for more then 3 months or so. This time I solved the problem of not having anyone to work out with buy purchasing a bowflex, now I won't have to quit and I can still push myself as hard as if someone was there with me. But here is where the questions begin.

I am 21 I am 5' 6" and weigh 150lbs. I currently am not "Fat" but I'm not tone either. I would say when flexed my abs feel like they are under about 3/4 of inch of fat hoever my arms and legs are pretty tone. I would like to lower my body fat and increas my mass. Should I attempt both of these at the same time? Or should I bulk up then lean down? Basicly for the past 3 or more years I have been eating lots of fast food and just whatever the hell I want. I am not "over weight" I'm just not where I personally would like to be so I would assume I really don't need to count every carb and everything I take in just make sure I am getting enough nutrition. What is the best way for me to build a nice body (chest, abs, back, bi's, tri's ect) I'm going for the whole package not just the "I wanna look fit" routine. I have currently been running a mile and a half once a week and I do rowing every day on my bowflex. The problem is I found that I don't get sore anymore.. I had been working out at the gym for the past month before buying my bowflex and I don't get sore anymore if I work out at the gym or bowflex. I feel the burn when I'm working out but no soreness? Could I be over working out or not working out hard enough? Also I have some back problems with my lower back I think that my lower back muscles are weak so which is better to do high weight low reps on it or low weight high reps? I just need suggestions to get me pointed in the right direction. I enjoy working out I actually want to work out everyday however I understand that this is not good for you that I won't make any progress. So what is the best? I have already switched up my diet some but I dont' want to work out for 3 months and feel like I haven't gained because then I"m scared I'll quit I want to work out 3 months and feel like I've gained lots and push myself harder. If someone would give advise that would be great. Also one other thing My chest never burns when I work it out. I can do bench press high weight low rep or low weight high rep and my tri's get sore and give out on me way before my chest is this something that I just need to keep working on until my tri's get big enough to sustain the stress and eventually my chest will be the one that gets sore?
 
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