New To Swimming

Welcome!  

Jammers (the longer form fitting trunks) are much better than loose swim trunks and will be a good investment.  

Lessons!  Learn good form now rather than trying to break bad habits.  

Given your description, I suspect the following:

You're lifting your head rather than ROTATING which causes your runner legs to sink and you're now plowing through the water rather than gliding over it (dropping legs are exaggerated by the additional drag caused by your swim trunks).  

You're holding your breath and trying to exhale/inhale in that brief second when you lift your head.  

The anxiousness of breathing makes you tense and you're thrashing around rather than using the water.  

Step 1:  Relax
Step 2:  Slow down
Step 3:  Get good glide position (practice looking down so your feet/legs/hips don't sink).  
Step 4:  R-o-t-a-t-e
Step 5:  To improve breathing, hum.  Exhale gently through your nose (which helps keep water out) with your face in the water.  

R-o-t-a-t-e

if your dipping your left shoulder, that's probably some indication that you're LIFTING your head (even a little bit) rather than rotating.  When your head comes out of the water, the rest of your body sinks.  If you're legs sink, try "looking down" further.  Since I can't see you swim, I'm assuming you're trying to look ahead as well as down and the water hits your face about your eyebrows.  Look more straight down with the water hitting near the top of your forehead.  In the extreme, look at your drawstring with the water hitting near the top of your head.  

There is a certain amount of natural body roll and your kick offsets much of that.  

No good videos to point you towards, but do your own "investigation" when you're at the pool.  
Channel your best 5-year-old imagination.  
Push off from the side and see how far you go.  Repeat 5 times making NO changes (same effort, same position).  
Now start experimenting.  Point your toes.  Do you go farther or not?  Repeat 5X
Tilt your head down more.  Do you go farther?  Repeat 5X
Changes to your hands/arms?  

Eventually, you'll have identified the things that give you the longest glide.  Now start to incorporate that body position into your stroke!  

Oh...and start to do some kicking drills.  Maybe 25 or 50 yards kicking only.  You can use a kickboard or flip onto your back.  If you go backwards kicking only, get some in-person lessons.  

Most likely, YES!  

Not sure about the knee pain.  

For the shoulder, it's common for beginning swimmers to keep their arms mostly straight (in fact, when I taught kids, I told them to do "windmill arms").  This puts a lot of stress on your shoulders (ok when you're 5 and doing it for 15 seconds at a time, not so good when you're older and trying to swim 1000-2000 yards).  A straight pull is also a common crutch for poor breathing.  

In a straight arm pull, your hand enters the water almost fully extended and "digs deep" in the water. 

Think about lifting a bucket of water using a line and an overhead pulley.  If you pull on the line with your arm fully extended, your leverage isn't very good and you couldn't pull well or hold it for long (and you'll hurt your shoulder).  If you step closer to the line, and pull it with your elbow bent and your hand right in front of your chest, you have better leverage to pull or hold the line.  

Swimming is the same, only instead of pulling a line, you're pulling a handful of water!  You want your hand to pass by your chest within about 1 hand-width.  

If you're standing on the ground making arm circles, you can fairly easily have your hands at 11 o'clock and 1 o'clock, but you have to contort quite a bit to get your hands at 12 o'clock (straight overhead).  That motion is hard on your shoulder, so you end up twisting your spine which really slows you down if you're moving through water.  

A proper arm motion has your thumb/fingers entering the water 11(left)-1(right) with your elbow slightly bent, flattening and extending fully just below the surface, and then pulling along your centerline within 5-6 inches of your chest all the way past your hips, exiting the water palm-up and full elbow extension, elbow bending as you bring your hand forward above the water.  Some people make a long "S" under water.  

L-e-s-s-o-n-s!!!