Be careful about using treated lumber has it is treated with toxins that will leach into the soil as it rains and degrades. Plants root systems and myccorhizal fungi will absorb it and it will get into your food.
Cgbg's use of stone and concrete footings is a good idea but the fencing behind and underneath needs to be thorough. There's nothing worse than finding something has snuck into garden and chowed down on your veg buffet. Plan to keep the hungry critters out now and you won't be disappointed later. Putting wood at the bottom of the beds instead of fencing will only rot in a few years. Another option would be to use landscape fabric, however my recommendation is still to go with the hardware cloth. If you think the soil is contaminated in some way, use the fabric. It's hard for animal, even those with sharp claws/teeth to get into.
In terms of starter plants, I always grow the food we eat the most of or is the most expensive in the supermarket. I used to grow asparagus but it took up a lot of space and after about 2 meals with it, I was done. OTOH, I can never get enough vine ripened tomatoes and it's easy to grow. I would also put peppers, green beans (both bush and pole), cukes, basil and most herbs, into the category of easy to grow because it doesn't attract many insect pests. If you're starting the garden now and are in northern hemisphere, you need veg that will grow quickly - eg cherry toms, lettuce, spinach, beets. If you are down south and have a hardiness zone of 8 or higher, you can get away with some of the larger veggies.
Summer and winter squash can attract vine borer so that requires constant vigil. Don't put herbs that will reseed or expand in your raised beds. Try pots for them, especially anything in the mint family.
Gardening has a high initial investment, but I've made back my money big time. My grocery budget is about $100 per month from June till October.