{\bf Abstract:}\,{\it This is an extended version of the first part of a forthcoming paper where we will study the local Zeta functions of the minimal spherical series for the symmetric spaces arising as open orbits of the parabolic prehomogeneous spaces of commutative type over a p-adic field. The case where the ground field is $\R$ has already been considered by Nicole Bopp and the second author (\cite{BR}). If $F$ is a p-adic field of caracteristic $0$, we consider a reductive Lie algebra $\widetilde{\go g}$ over $F$ which is endowed with a short $\Z$-grading: $\widetilde{\go g}=\go{g}_{-1}\oplus \go{g}_{0}\oplus \go{g}_{1}$. We also suppose that the representation $(\go{g}_{0}, \go{g}_{1})$ is absolutely irreducible. Under a so-called regularity condition we study the orbits of $G_{0}$ in $\go{g}_{1}$, where $G_{0}$ is an algebraic group defined over $F$, whose Lie algebra is $\go{g}_{0}$. We also investigate the $P$-orbits, where $P$ is a minimal $\sigma$-split parabolic subgroup of $G$ ($\sigma$ being the involution which defines a structure of symmetric space on any open $G_{0}$-orbit in $\go{g}_{1}$).}