The above picture shows a simple 4 foot by 4 foot raised bed frame. This was made from two 2x6x8 foot boards, each of which was cut in half and screwed together. When factoring in the average reach of a person (about two feet), this keeps everything within easy reach and avoids unnecessary physical strain.
You can also use a rectangular shaped raised bed to make a raised bed easier to work. Two 8 foot 2x6 boards are all it takes. Cutting two feet off each board and attaching them together gives you a 2x6 foot raised bed for your garden. Depending upon what you're planting you can use 2x8's, 2x10's, or even 2x12's to make a raised bed that will provide deeper soil levels for your plants.
It also helps to add some additional bracing inside the frame for your raised bed. A simple 4" corner brace on each of the corners of your raised bed will add additional stability. Using basic sizes will also make them easier to move if necessary and allow you to place them according to available space. Old newspaper or a cardboard box can be used as weed paper for your raised bed.
It doesn't require a lot of material to build a decent raised bed for gardening but you'd be wise to keep basic sizes in mind that won't cause undue physical stress by putting things beyond your normal reach. Gardening can be a very rewarding effort if done right.
Be aware. Be informed. Be prepared.