Hey
We've got an allotment - had it for 7 years now. Ours is one rod by 6 rods long - so 15ft by 90ft. We have a shed, greenhouse and compost bin on it.
My suggestion would be to only grow the things that you eat - sounds obvious but DH and I keep growing cabbages which we would never buy.
We can't grow enough peas and beans.
It's also to put in some permanant crops as they tend to need less TLC - we've got red, white and black currants; gooseberry bushes, rhubarb, asparagus, globe artichokes, strawberries, raspberries and hops.
In terms of stuff to be planting now i'd get some onion sets and some garlic - they can go in now and are fairly low maintenance. Also parsnips if you like them.
Then if you've got a warm windowsill you can plant outdoor varieties of tomatoes to get them germinating.
Squashes are fun to grow.
Leek seeds need to get planted next month - for next winter.
If you're going to do potatoes you should buy seed potatoes soon and get them chitting - that is sitting in a cool bright place to get started before you plant them.
In terms of tips don't be tempted to plant tings too close - we ALWAYS plant our potatoes too close and then don't get as good a crop as we might otherwise.
HTH :)