I can't really say how much you need to lose.... it's a very individual thing. It's also a touch & feel process.... meaning you're not going to stop just b/c the scale reaches a number you've been shooting for. Instead, you're going to let the reflection in the mirror guide you. When you're happy with that, that's when you move to maintenance.
So yea, I'd agree that work on the weight loss and like I said above and this will drastically help you reach the level of being able to do a pull/chinup.
In the meantime, focus on full body training that will include emphasis on the prime movers involved during the pull/chinup.
I'd do something like this, and this certainly isn't written in stone:
Workout A:
Dumbbell Squats
Romanian Deadlifts
Dumbbell Benchpress
DB Rows
Direct Bicep exercise
Abs
Workout B:
Dumbbell Deadlifts
Dumbbell Bulgarian Split Squats
Overhead Dumbbell Press
Chinups with negatives
Shrugdowns
Shrugbacks
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With dumbbell squats, only use dumbbells if you're are proficient with squatting just your body weight. Here is an example video or two:
Dumbbell Squat -- I'd actually like to see you get a little deeper than this in the squat.
I'd also read through this entire thread as it goes through some very important tips with regards to some of the above exercises, including the squat:
http://weight-loss.fitness.com/weight-loss-through-exercise/14264-basic-lifts.html
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With the Romanian Deadlifts, you don't want to keep the knees locked. You want a slight bend in the knees, which is maintained throughout the movement. This takes the stress off of the knees.
You want to keep the natural curvature of the lumber (lower) back. If you find that you are rounding your lower back, drop the weight until you can perfect the movement. You will be able to get "deeper" in the movement as your flexibility improves. For now, the bottom of the movement is just above the point where your lower back wants to round. Full range of motion for the average person usually allows the bar to travel just below their knees.
You want to keep the bar close to your body (hips, thighs, knees, shins).
A huge misconception people have is, you simply bend forward at the hips. They are missing out hugely. As with the squat, you want to initiate this movement with your hips, by sticking your butt back. Pretend there is an imaginary wall behind you that you are trying to push your butt against. You don't want to go so far back with your butt/hips that you lose balance. Just before that point, you can start flexing at the hips, which brings your torso down toward the ground. Descend until you feel a stretch in the hamstrings and glutes OR to the point where your lumber wants to round (as noted above).
To reverse the movement, simple extend your hips and bring your torso back up. I like to overly extend my hips, basically thrusting my pelvis outward and squeezing my glutes at the top of the movement. Follow me?
It is very important to keep your back fixed, as the torso leans forward.
Romanian Deadlift
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With the DB Bench Press, You want to keep your upper back tight by retracting your shoulder blades. Think: Pinch my shoulder blades together. On top of pinching them together, also depress them toward your feet. Basically, you are pushing the shoulder blades back and down.
Your chest (rib cage) should be pushed out, which will happen automatically when you do the above with your shoulder blades.
Your feet should be set flat on the floor. They don't move from this position. I see many people benching with their feet up on the bench. Some people even keep their feet up in the air. Don't do this. Put them on the floor, gain your stability, and keep them locked there.
For general benching, you want to maintain the natural curvature of your lumbar spine. There are three points touching the bench and four points you should worry about. One is your head. Two is your upper back (shoulder blades and traps). Three is your butt. And four is your feet on the floor.
Be sure to read the blurb about benching in this thread too:
http://weight-loss.fitness.com/weight-loss-through-exercise/14264-basic-lifts.html
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With Dumbbell Rows, this guy gives pretty good instruction actually:
YouTube - Dumbbell Row Exercises: Strength Building Workout Routine
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With direct arm work, I could care less what you do. 1-2 sets for biceps and triceps will do.
Same goes for abs.
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With the DB Deadlift, be sure to read the blurb about them in the aforementioned thread:
http://weight-loss.fitness.com/weight-loss-through-exercise/14264-basic-lifts.html
And here's a decent vid of them,
here.
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Here's a vid of DB Bulgarian Split Squats,
here.
Add dumbbells when ready.
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Overhead DB Press is very simple, but be sure to read the blurb on it in that same thread:
http://weight-loss.fitness.com/weight-loss-through-exercise/14264-basic-lifts.html
Here is a vid, but I'd rather see you do them standing.
Stability Ball Shoulder Press Exercise
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With negative chin ups, all you are doing is taking a chair over to your pullup bar and taking an underhand grip on the bar. Use the chair to start in the 'up' position. From there, let your body come down slowly, maybe for a count of 5 or so. At the bottom, put your feet back on the chair and start back up in the 'up' position.
We are stronger eccentrically than we are concentrically. Fancy way of saying you are stronger when the resistive force is on the way down opposed to the way up.
I don't have a vid of this, so if you have questions, let them fire.
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Shrugdowns and shrugbacks are simply pullups and rows, respectively, without the arm bend.
So hanging from the bar in the pullup position with the arms extended, you'd simply shrug the shoulders straight down. It's a very short range of motion.... I really wish I had a vid of this. Damn I need to get my site up and running!!!!
Shrugbacks are simply the rows from above with out the arm bend. Instead of bringing the DBs up to your torso, you'd leave them hanging and simply shrug your shoulder blades up towards the ceiling. Again, it's a very short range of motion.
In both exercises, the arms stay full extended throughout the exercise.
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So that's a lot to sink in if you've never really worked out before...... read through it and let me know where you have questions.
Once things are clear, we'll discuss sets and reps.
You might want to take a few days and toy around with the exercises to find questions as you try.