There are many possible approaches here.
You are saying youy want a H0 scale layout on a 4x8 foot table, with a "yard" and a "city".
There are three challenges her in your request:
1) The choice of a rectangular 4x8 foot table for the layout
2) What you mean by yard, and
3) What you mean by city
1) The stereotypical H0 scale layout with a loop of track on a rectangular 4x8 foot table has several challenges in terms of fitting in a track plan onto the table, and in terms of how much layout facilities you get relative to how much floor space you use for the layout and adjoining walk-around aisles.
Some people feel it is too early to introduce these concepts to a beginner, others (including me) feel that it is worth telling people about these challenges early - and then letting them decide for themselves whether they still want to do the 4x8 foot thing, or whether they are able to learn from the experiences others have made about the 4x8 foot footprint.
Follow this link to learn more about challenges with the 4x8 foot approach, and to see some alternative approaches : http://www.layoutvision.com/id28.html
A good way to determine whether the 4x8 foot footprint is good for your purposes or not is to measure and describe the entire room you intend to have your layout in, and describe how you want to fit the layout into the room.
Surprisingly often (but not always), using a different footprint can fit in a lot more layout in the same space, while negating some of the challenges the 4x8 foot format has.
For instance, in my basement I have a room too small (it is 6.5 x 11.5 feet big - just under 75 square feet) for the traditional 8 x 10 foot space a 4x8 foot layout with 2 foot wide aisles on three sides need.
By doing a doughnut design with a narrow lift-out/duckunder across the door instead of a traditional walk-around 4x8 foot rectangle, I was able to fit in about 44 square feet of layout (compared with the 32 square feet of layout for the traditional 4x8), still have ample aisles, still being able to use the room for other purposes, and aving room for a lot more railroading than what would have fitted onto a 4x8 foot table:
Doesn't mean that this specific way of doing things will necessarily be the best for you. But it may be worth it to explore what your room looks like, and what your goals are.
2) A yard can be many things to many people.
Some people basically mean "a couple of parallel spurs where I can park two trains, so I can choose which of the two trains I send around the loop". This parking of a whole train, before it makes it's run, or after it has made it's run, is often called "staging" in model railroading terms (waiting to appear on stage).
Other people want to model how the railroad moves cars - and want a small yard to do something in the yard - like to sort RR cars into groups ("blocks") of cars that are sent in different directions - say "eastbound cars for the local", "eastbound cars that goes to the next yard down the line and beyond". "westbound cars for the local" and "westbound cars that goes to the next yard up the line and beyond".
Or to have a small yard that is used as temporary work space for trains that deliver cars to local industries - a couple of tracks where inbound (arriving) cars can be left temporarily while the locomotive is picking up outbound cars from the local industries, and then the outbound cars (that has been picked up) can be left there while the locomotive is spotting the inbound cars at the local industries.
Or to have a small yard which essentially is the home base of a short line - where they park a couple of engines overnight.
Yards can mean different things to different people. Do you have a rough idea what you want to be able to do with your yard?
3) Same with the city. Space is at a premium with a small layout (and the 4x8 is a small layout, despite taking 80 square feet of space when being used). Cities can mean different things to different people.
If you just mean you want a background that looks city like (lots of buildings, maybe a grimy industrial look), that can often easiest be achieved by using a trick known as a backdrop/viewblock - modeling only one side of the buildings or partial buildings, and using some kind of board to block your view of the buildings from the opposite side of the table.
But there are lots of approaches to model railroading. Rich pointed you to the toy train track plans at the thortrains web site - which also is a perfectly fine way of starting, if these plans give you what you are looking for.
What I have tried to do above is to give you a brief introduction to the level beyond "a ready made trackplan for a 4x8 foot table".
Whatever you do, don't get intimidated by the number of possible approaches. Doesn't mean that you have to explore these ideas I have introduced - although I still recommend reading Byron Hendersons web page on the 4x8 foot footprint I linked to above before you dive into a 4x8 foot layout - the page also contains links to a couple of pretty good (from my point of view) 4x8 foot track plans.
It just means that this is a hobby that can give you a lifetime of enjoyment - it is not just a loop of track, watch it run for an hour and then moving onto another hobby.
Welcome to the forums :-)
Stein, a sometimes overly talkative guy who tend to say "there are many possible approaches"