rule #1, dont stretch before you work out, makes you more prone to injury
now, because you havent worked out in awhile, first week, you're gonna be sore as hell,its totally normal.
1 hour before working out, get 30 grams of protein and 60 grams of complexe carbs (oats,rice,etc... no sugars). this will help your muscles get ready for the intensity.
now,here's a little trick. it might sound crazy,but it will definitely help with muscle repair(less soreness too) and protein synthesis. Half way through your workout, drink about a can of coke or pepsi(not diet) as quick as you can, then finish your workout. this will essentially refill your muscle glycogen so you dont lose intensity and strength. your muscles will be on track doing their work. plus,you will feel orgasmic pumps
half hour after you're done workout out, get 30 grams of protein,and some simple sugars(yes sugars, so add sugar to a protein shake,or eat some cereal). NO FRUITS because those sugars are fructose and wont do a damn thing to the muscles.
follow this plan everytime you work out,and i guarantee soreness will not last that long. you must at least use this advice the first week because thats when you experience the most soreness.
if you've been training for awhile and you dont feel soreness, you need to up your intensity. soreness is a sign that you really worked them muscles.if you dont feel sore everyday after you workout,you're doing something wrong.
make sure you get enough sleep,muscle wont repiar itself while you're still working them.
last piece of advice. never work the same bodypart 2 days in a row. if you did biceps today, dont do them tomorrow. a bodypart needs 24-48 hours to repair itself enough for another workout. if you dont give it enough time,then this thing called muscle wasting happens. your body will say fuck this,this muscle is too damaged, just scrap it i dont have time for this. so you will lose muscle this way.
good luck