Vegetables: potatoes, carrots, cabbage, brocolli, peas, sweet potatoe, parsnips, cauli, mushrooms, cougettes, peppers, onions, cucumber, tomatoes, lettuce
Don't bother (too difficult / not worthwhile for a newcomers): sweet potato, mushrooms
Now / urgently: potatoes, parsnips. Too late for Onion seed, but Onions from sets maybe.
If you have a greenhouse then raise plants indoors (.e.g. on windowsill / conservatory) to plant in the greenhouse in a month's time: cucumber, peppers, tomatoes - but its getting late for Peppers - considering buying plants in Garden Centre instead - and Toms and Cues too if you like / can afford! Wait a while yet though if you are buying from Garden Centre otherwise you will be trying to keep them alive on your kitchen windowsill!
Don't bother with peppers if you can only grow them outside, and make sure you get an outdoor variety for tomatoes, cucumber (personally I don't like outdoor "ridge" cucumbers.)
Sow a few every fortnight: carrots, cauli, lettuce. Note that Cauliflower is considered "difficult". I don't find it so, but I sow literally 4 a fortnight; their harvest period from "ripe" to "ruined" is 2 weeks at best, so sowing a whole row is only any use if you want to freeze them or give them away!
I don't know anything about Peas (but I would guess that "now" is the time!)
cougettes: Sow in pots, inddors, one month before last Frost. They hate the cold, so be conservative. I reckon 1st June to plant out, even though we haven't had a frost in 2nd half of May for years. So I sow on 1st May.
cabbage, broccoli: I don't grow Summer varieties as I consider these a Winter Veg (and lots of other things are available in the Summer). But the Winter varieties need sowing Soon / In-the-Summer, so need planning for now.
Fruit: we like any sort of fruit so any that are relatively easy to grow.
No quick-return here that I can think of. Strawberries will probably give you some crop this year, Raspberries might if you choose an Autumn variety; for a Summer variety you get nothing/little in the first year; Apples etc. generally even longer. I have 10 varieties of Raspberry from "earliest-early"
right through to one that barely fruits as the frosts arrive.
I'm also after where to get my seeds from, is it best from your local garden centre or on one of those seed websites?
Given your starting point I would recommend a garden centre. Allow at least an hour! sit and read the back of the packets, make some notes about what to sow when, and when it will be ready for harvest so that you can decide what you need. Do NOT get too carried away - 10 varieties of anything is NOT required!!! (In fact 2 varieties of most things is overkill).
As a Newbie my most important recommendation is to grow what you like (no point growing something you hate, have it fail, and then think "Why am I doing this"!!)
Also, grow what is relatively expensive (e.g. Early Spuds), or "tasteless" (Tomatoes), in the shops
e.g. don't bother with Main Crop spuds if you have a small plot - they are 10-a-penny in the supermarket in Winter, and better to use your land for growing "fresh" produce, rather than growing to Freeze / Store - what you grow to Freeze / Store will taste little/no different to the Supermarket.
However, there are a couple of things you have not included in your list that you might want to consider:
Summer beans: Runner Beans are the most popular (and prolific), otherwise Climbing French or Dwarf French. Plus Broad beans?
Sweet Corn? Particularly the SuperSweet varieties such as "Swift". Must be in the pan within a minute or two of picking, but it tastes like "heaven" compared to shop bought.
Also consider what you will grow for the Winter: Brussels, Leeks, (Parsnips are on your list), Spring Purple Sprouting Broccoli ("PSB") as well as the Autumn "Calabrese" Broccoli
There is a calendar of when=-to=sow and when-to-reap here:
http://www.allotment-garden.org/vegetable/general/sowing-harvest-vegetable-chart.phpIf you are a detail-person, and like fiddling with spreadsheets etc., I have one for crop planning which you are welcome to. Details here:
http://chat.allotment-garden.org/index.php?topic=34306.msg411041#msg411041