To center your rifle:
Start at 25 yds with your rifle on a rest or steady on a table, I use an old sawhorse. Fire 1 shot directly at the center. If you feel like your shot was well aimed and there was no breathing problem or twitch with your shot, then don't fire another.
Now, look through your scope and put the cross hairs on the bullseye and while you steadily hold them there, have someone else adjust your elevation and windage until your cross hairs are on your first shot hole. At this point your rifle is centered.
To Zero your rifle
Now, move back to 100yds and turn your scope's magnification up as high as it can and take three very steady and comfortable shots. Go to your target with a black sharpie marker and draw a triangle around the first three shots. Now, measure in inches how far you are off from bullseye and then do the calculation 1/4 of an inch = 1 click. So if you are 2 inches to the right and 1 inch high, you'd move your scope down 4 clicks and to the left 8 clicks. Then repeat the process. Fire 3 more shots, go mark them with a triangle and repeat until you are satisfied with your grouping. Use a great deal of patience while doing this! Also, make 100% sure you are steady when each shot is taken. I recommend using a table where your back elbow can rest and your front elbow can rest AND have a sand bag to rest the forestock on.
Hope this helps!
...and on that day when my strength is failing, the end draws near and my times has come, still my soul will sing Your praise unending, ten thousand years and then forevermore!