It all depends how you define "good".
If you were to buy it pre-built, it'd cost a fair bit more than physically building it yourself.
Alternatively, you can have a custom-built PC - most manufacturers offer this - but again, actually buying and building your own components is usually the best option, if you can.
If you let us know a rough budget, what you need, and whether you are comfortable with building your own PC from scratch or jot, we can let you have some suggestions.