Is this an evergreen Hibiscus, Marleen? If so, I would choose the straightest branch from near the center of the bush, stake it up straight, and cut all the other ones back by about 1/3, preferably to a side branch. Keep cutting the "rejects" back several times a year, but let "the chosen one" grow as much as it wants. After a couple of years of this, the main stem will have enough of a crown to support strong growth, and you can cut off all of the branches from the base. After that, keep an eye out for sprouts at the base and along the trunk, and rub them out before the get woody. Cut lower branches off as necessary to make a taller trunk, but without removing more than 1/5 of the foliage.