Perfect Loop

I have a guitar sample which is about 20 seconds long, I want to record it to a track so that it starts right and the beginning of the bar and ends exactly at the end of the bar, so that it loops seamless and perfect. Every time I try it is either overlapping or to much of a blank space in between. Is there a recommended amount of bars? i dont know if I should use 4, 8, ?. or do I have to adjust the tempo or tsig. I am new to this and I do not quite understand the relation between bars, tempo, and tsig, Could anyone help me out? I have tried for hours and connot figure it out so I'm sorry for a question so basic.

I wouldn't try to loop a 20 sec. sample. experiment and see if you can chop it down alot go to trim screen and look at your waveform then start scrolling your begging and end till it starts to sound decent once you get it to a point where it is tolerable the go to the loop screen and start fine tuning it. Remember to go to the loop field and set loop to on.

I chopped it down to 15, but I still cant get it to loop perfect.

post it up in 16-bit 128kbps wav format so i can see what you need to do.