A simple 18-55mm kit lens will be enough for the beginner to begin practicing landscape and portrait photography. For most, the lens solution for landscape and portrait photography involves investing in two different lenses. For example, I personally prefer an ultra-wide zoom such as a 10-22mm lens for landscapes, and a telephoto zoom like a 55-250 for portraits. Certainly there are many, many other choices that could be a better option for you depending upon your needs, expectations and budget.
As you shoot, you will find out whether or not your gear is allowing you to get the kind of results that you want. You could find that the 18-55mm kit lens to be good enough. It's only through use will you determine if your existing gear is meeting your needs and expectations. For example, you may find that the 18mm setting is not wide enough for landscapes and the 55mm position not telephoto enough or that the f/3.5~5.6 aperture not large enough to provide you with the light gathering capabilities or shallow depth of field that you may desire.
If you can explain what gear you have and what about your current lenses that isn't working for you, then we can provide better options.