Don't do it! It is too much stress on your hair.
Black hair is very fragile and needs to be treated as such.
I would not relax, there is no need if you are going to weave it. I recommend a good deep conditioner treatment and trim the ends if need be.
Another thing as well, relaxing the hair line and the section that will be lefted out...will cause breakage. I have seen many sisters with short and uneven hairlines because of this process. Get a hair closure from one of the vendors and protect your hair.
We all love weaves but, not at the expense of own natural hair