We are into Vipassana the original path of Buddha. In this we start with focus on breath then all over body for sensations and finally within brain & spine.
1) If you have sat down for mediation you are meditating , tell tale sign is whether mind is wandering , if yes , stop thinking and start focusing again w/o lamenting about lost time.
2) Get up , pace around with slow walk w/o losing meditation , meditation is not about endurance nor about striking a pose.
3) Dont, you may say its strange or contrary but the fact is that mind never gets focused permanently (it does only in trance state ) . Rather mind gets focused only for a time which increases slowly with practice. So meditation is like a lawn, as new grass grows , mow it again & again. As new thoughts come up , kill them all and start again. This is the only way. Remember focusing mind is not the only objective, the real objective is to become a better person by removing bad emotions & tendencies like greed, anger, hatred, jealousy , passion, lust etc. and you do that step by step whenever the mind gets focused by eliminating thoughts because thoughts are the vehicles for emotions.
Specifically , if you want to make mind focusing easier, follow these 5 moral principles of Buddha , no killing(nor hurting) any, no sexual misconduct(celibacy is best), no lies, no stealing & no intoxicants. These moral principles make your mind cool down (not racy) & calm naturally. One secret, Sex has very bad effect on mind and within 24 hrs you surely have uncontrollable thoughts & lusts , dont believe it, try it & know.
4) Simple, the longer the better, minimum 20-30 minutes and max any. The longest you do the faster results come and dont worry there is no bad effect of any Buddhist meditation (about others I cant say).
But long meditations may make you get visions/hallucinations , but dont worry , its a sign of progress .
5) Depression :
Depression is a clinical condition. When a person feels threatened repeatedly or due to other emotional reasons , depression may develop. There is a thing in the brain called Amygdala .
http://en.wikipedia.org/wiki/Amygdala
Amygdala can get activated due to various social , emotional activities. When Amygdala starts misfiring then it releases chemicals which make you depressed.
Sexual activity or drinking alcohol/intoxicants may also lead to temporary depression after few hours because both affect brain so reduce these as much as possible.
How to get out of depression using meditation ?
I suggest you do this 10 day Vipassana meditation course (its tough) in a retreat near to you first :
http://www.dhamma.org
Hopefully your problem will evaporate but if not then do this (both together) :
(a) Do the same meditation every day , find time for it .
(b) Another important thing is to meditate within your CNS (central nervous system) i.e. within brain & spine , normally you cannot access it , not even with the meditation initially (because this is the ultimate objective and hard to do) So use this method - focus on top of brain and try to move inwards within brain , it will appear impossible to do but persist , try , try & try , it may even take days, weeks, months or years but dont lose patience because it is possible , many achieve it as I did too. So when you do it spread your awareness within entire brain and then spine , now you will feel a great bliss , suddenly you will feel as if you touched a live wire, this is electrifying experience accompanied with a great bliss . THIS IS TRANCE. So you will enjoy this trance and it will go on for a long time (may be hours) . This will completely change you and your perception about life because you have found the Holy Grail hidden within you , the unlimited source of happiness. And you will try again & again to attain this state of trance .
After experience of this , looking back, you will realize amazingly that depression is gone away for ever.