You have to spend with this person several month together physically (but non nesessary sexually) to learn more about the subject of your love and to find if you have common interests, and if you are tolerant to each other's habits, tastes and some other strangeness every human may have.

If after this you will still STRONGLY feel that you want to have this person (with all her pluses and minuses) be around you FOR ALL YOUR LONG LIFE (quite a commitment by the way) AND if she will still feel so too, then you two may decide to strengthen your bonds by formal marriage (or some other similar act).

I think, you do not want to marry somebody in a rush to only discover a few month later that it was a mistake because you did not know your party well, right?