In general it should be someone who leads a Torah observant lifestyle. Very often Rabbis or people who have been instrumental in the growth of the bride and the groom are asked, since it is considered to be a great honor.
The simplest thing is to tell the person that you would like to honor him with a blessing under the Chuppah, and ask if he agrees.
The first two blessings are recited by the same person. Therefore, I advise that the couple apportion the blessings carefully since a maximum of six people can recite them.