1.Can i use normal computer headphones with the pod because the studio monitor type are really expensive for me so will i be able to get good sound through computer headphones(I'm not asking about earphones)

Yep, but be realistic.  The more you invest in decent headphones - i.e. ones that faithfully reproduce the tone you have dialed up rather -  the better your tones will be when you try them through a PA or similar.  Anything is better than earbuds.  If you just want it to sound good and arent too worried about using building tones that are 'portable' then any headphones will work.  You do get what you pay for though, with headphones.

2.I've a spider 4 15w amp and i know it doesn't work well with multi fx pedals because its an amp modeller itself and i tested my friends vamp3 with my amp and didn't sound good.I tried putting the amp in neutral state by pressing the tap button as i power on the amp though it worked but still i didn't get good sound through the amp with that vamp pedal and also tried the amp in clean channel and didn't sound good too(should i put everything to 0 on my amp or should i put the treble mid and bass at centre?)

The spider 15 is a cheap and cheerful practice amp.  That is fine but it is limited by the small speaker and construction and it is not going to sound 'huge'.  With the amp in a neutral state you would select 'Combo Power Amp' or 'Stack Power Amp' normally on an HD.  The amp's tone controls would not work at this point so all editing would be done from the Pod.

There are many reasons why the Behringer may have sounded poor ranging from patch settings to output mode to whatever.  I would not use that as an indicator as to how good a Pod may or may not sound.

3.how can i use my amp just as a speaker so it doesn't colour the sound of the pedal?

By doing what you did ... i.e. to bypass the pre-amp using the tap switch/power trick.  You are not bypassing the 'amp' ... you are bypassing the 'pre-amp'.  You will still use the 'power amp' section of your Spider 15 ... you need this to amplify your guitar signal up to levels where you can hear it through the speaker.

If that doesn't sound good then you could try powering up the amp as usual, selecting a clean channel and centrering all tone controls.  There are tweaks in the POD setup for running 'Combo Front' also.

4.How can i connect the pod through the cd/mp3 in of my amp.Should i use 1/4 to 1/8 adaptor? how does that work? and what is a dummy jack plug? i read some where that if i use this method i should also use a dummy jack plug.

If your amp has an mp3/cd in then you are effectively running it the same way as you do by starting up with the tap switch held down ... i.e. bypassing the pre-amp.   If you still want to try this:  you could either run from the headphones jack on the HD into the MP3 jack on the Spider (using a 1/8" stereo TRS into 1/4" Stereo TRS connector).  EDIT:  the dummy plug is required in the Spider input socket, as per Nick who posted up while I was drafting

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In my honest opinon though, I don't think that adding a POD HD to your Spider is going to revolutionise the sounds that you get.  They should be better than the spider alone, yes, but the Spider itself has many limitations even when used with the POD.   You may find that you get a very small improvement for your money.  If you are looking for good practice sounds then a POD and headphones may be better and invest in a bigger, more capable amp later.