What kind of space? How many people? What's your budget? What are your tastes? Do you have help organizing and hosting it?
Figure out your priorities, and allocate a budget and time/energy expenditure to each. Decorations, costumes, food, booze (if you're of age), games.
Figure out a date that most of your friends can make it. At this time of year, this may be your biggest challenge, and what you should address first so you can figure out a head count. Knowing how busy October is for most of our social circles, we claimed Oct 18 for our party back at the beginning of August.
For decor, there are lots of options - Party City, Spirit, Amazon (especially if you have Prime), dollar stores, Big Lots. Figure out a specific theme that fits your taste, and stick with it - that way you can grow your collection over the years.
Don't be afraid to start kinda small. Our first Halloween party had 8 people, last year we had 50(ish), this year we'll probably have around 40.