Most people suggest lens around the 100mm as being the "best" for portraits, some use longer, it's something of a personal choice and a lot depends on your location. If your in a small home studio you might not have the distance for a longer-ish lens.
Your question is so very open ended that no one can give you a specific recommendation!
As mentioned above more insights are needed.
Perhaps as a first step is for you to post some of the portraiture you have photographed with your current D5300 and the 18-55mm lens .....why, you may ask, should you show us your pictures? Well, without seeing what your portraits look like now any suggestions for a "better" lens will be a waste of your money and a 'waste of breath' of anyone's advice.
This is a photography forum...........so why not post some of your portraits for feedback & constructive criticism. There is in the sense photography genre no such thing as a better lens but when asked in the context of:-
These are my pictures......how can I improve them based on that they were taken this camera & lens?
Based on the feedback only then is the logical question as to whether a different lens (note not a better lens) may help you improve the type of images you have already taken or those you want to take in future?
You use an 18-55mm lens so you are using a crop sensor camera. Depending on how much of the person you want to get in the frame, you could use something as wide as the 35mm Professor1991 above recommended for a whole body shot, however generally you're better off using a longer focal length. On crop I'd probably suggest between 75mm and 100mm or even a little longer.
Why not post links to the kinds of pictures you'd like to take.