I wish I was in your shoes - being naturally lean and wanting to add mass. You describe yourself as an ectomorph and there is a clear path forward for your body type. The first thing you must realize is that which exercises you do are secondary to your gains. First and formost is diet; expecially in the area of protein intake. For an ectomorph, a good rule of thumb is 1.5 to 2 grams of protein per pound of bodyweight. Since you weigh 146, shoot for 250 gms of protein per day (middle of the 220-290 range) with copious amounts of good carbs (400-600 gms per day). I wouldn't worry too much about fat content at this point since you are naturally lean, but don't overdo the saturated stuff. Use protein supplements (whey/casein) generously, to get the protein levels up. Trust me, its hard to eat that much protein naturally without supplements.

As far as workout routine/exercises that is really up to you where you want to build (just don't neglect legs like so many guys do). The only thing is, after a adequate warmup, go HEAVY HEAVY HEAVY - to the point of pain (no pain no gain). Use a spotter with free weights. You won't believe just how heavy you CAN go until you break through your mental block. Go heavy enough that you fail in the 6-10 rep range. 2-3 sets for each bodypart is all you need (the first being a warmup, the second or third the heavy sets. Don't work a muscle group more than twice a week with several days in between for recouperation, muscle growth. There has been renewed interest lately in several individuals like Casey Viator, and Mike Mentzer, both of which were champions) who only worked out ONE set per bodypart twice per week, with extremely heavy weights. For building mass, basic exercises are called for like bench presses, squats, barbell curls, leg extensions, etc. (All the talk about 3 exercises of 3x10 per bodypart, is great if you are shaping and chisling what you already have.) Thinking back on my progress over the past years I am re-incorporating this BACK into my routine. If you tire of the routine - change it by adding/deleting exercises. (tired of doing squats? - do leg presses.) Just keep the poundage way up. And, when time to add weight, don't add a just a couple of pounds - be generous. Be SURE to have a hi carb protein shake immediately after (within 30 minutes) of each workout. your local gym probably has them for sale.

For best results, use a good creatine supplement, and add a good prohormone suppliment like 19-nor-DIOL (not dione), or 1-AD since they have little to no adverse estrogenic effects. If you can afford it, I highly recommend (and I usually don't recommend Brand names) MAG-10 from Biotest. Its primary active ingredient is 1-test (1-AD). I used it recently for a 6 week cycle (2-weeks on, 2-weeks off, 2 weeks on) while eating cleanly, and put on 15 pounds of weight, while losing one pant size. This means I probably put on close to 20 pounds of mass while losing maybe 5 pounds of bodyfat. I also worked bodyparts only twice a week (but because of tendon problems couldn't go as heavy as I would have liked on some movements). I did this at age 50 - you should be able to exceed my results. The only drawback to this stuff is price. Biotest charges $99 per bottle (which is good for 11 days, when used twice per day), but usually offers it on a buy 2 get one free basis - which should get you through the cycle I did. There are a few 1-AD supplements on the market now which costs less, but I can't vouch for their effectiveness as compared to MAG-10.