Why have one book of the series but not the others? It doesn't make sense.
If you can afford to buy them all, or like myself, are lucky enough to have someone buy you the whole set for Christmas, then great, have them all. If, however, you can only afford one, or you only have time to read one, I think it makes perfect sense to actually choose the one you actually want. If you have read LotR, the Silmarillion and UT, it will make very little difference to you which order you actually read HOME in. They are not like novels that need to read in order. They are more like historic texts, showing the evolution of Tolkien's writings. Due to the fact that the later three books contain drafts that JRR wrote after he had finished writing LotR means the stories are closer to their final form, and also include material that never even made it into the Silmarillion. These three also contain material about Elven culture, languages and life/death/rebirth that you will not find anywhere else.
I still stick with my original advice. If you want to go further than UT, Morgoth's Ring would be my first choice. Don't let me put you off getting the whole collection though, Loni. Let me know when you have finished them.