Could someone explain to me how to use, compression or whatever to make uneven volume takes in the same track come out as an even volume. This happens because I do not remember the volume of my guitar or bass when recording and doing different takes on different days. Any help much appreciated thanks.

Use the volume automation feature. If your new to compression, that would be the easiest.

It has taken me quite a while to grasp the correct concepts needed to be able to use compression effectively.

Compression is a really bad tool for this. Any compressor will vary in its effect on tonality depending on how hard the signal is being hit. So you might well even out the volume, but end up with strange effects downstream.

I personally wouldn't use volume automation, either. If you do it with clip gain automation, that comes right at the start of the signal path. So any compression dowstream from there will get hit more evenly by the incoming signal.

The amazing thing is that I rarely touch and adjust the volume or level controls on my interface or anything else in the studio.  I do play with the POD2 levels.
 
Compression is not the best option.  It has it's uses but getting to know how to set you levels to be consistent is more important to getting the levels the same. 
 
If you are working an a song and think that you might need to come back to it later, I would suggest using a notebook. Put each new song project on a new clean page and use as many pages as you need to make notes on the various FX, VST's, mic levels, amp levels, etc.... so that if you do have to come back and do a new take a few days later, you can quickly dial in the settings you were using.
 
I do not follow my own advice on this and have in the past, found that I could not figure out the settings I used to get a certain guitar tone.  In that case, I had to redo the entire track to make things consistent. Lots of work, but, I also have the belief that if I played it once, I can always play it again and often better the second, third, or twentieth time through. 
 
But yeah, keep notes at least until you get a certain consistency about what you're doing in regard to levels.

there is a GAIN function, at the clip level.
 
you COULD go thru, put breaks between the clips where the volumes are different, tag JUST the clip you want to bring up or down, and apply positive or negative gain to JUST that part of the clip you want to match to others.
 
 
but the FASTEST way to do it, is just to use a volume envelope, and just simply mix it.
 

Lots of good suggestions. Another option is to use the gain command:
4. If you want to switch the two channels, reverse all four sliders from their present positions.
 
 

I would've mentioned something like bluzdog's method, with Normalize instead (but it's sorta hard to use), that's always been my favorite quick, clean, and painless method- much easier and less of a mess than automation in my case

I mostly use clip gain automation to even out, say loud and soft parts of a vocal track for instance. The clip gain is pre f/x bin so then you can use a compressor for the final touches.

+1 on clip gain to alter the volume on the various takes you have.
This allows you to use other effects like volume automation later on in the project. I would clone my tracks before this though so you can always go back to your original recording if things don't work out.
Also +1 on keeping notes. There is a page for writing project notes within the Menu bar, or even within the track inspector itself. It's good studio practise and you'll thank yourself for it in just these situations.