nice shots for a newbie
for a start, to prevent handshake, one would usually use a shutter speed of 1/focal length of faster (that is: if u are shooting at 250mm, u shd try to 1/250s or above) however, with the 4-stop IS built-in into this lens, u can shoot at lower shutter speeds, but ur holding/standing and shooting posture shd be stable to get optimum stability.
for most of the shots, i see that they are composed with subjects dead centre. try different framing and composition techniques to improve the visual impact of the photos. can read up on re-composing/framing/rule of thirds/etc to improve.
cheers!