Helen: maybe these will help
You can paint the wheelbarrow with paint that matches its constructions, say wood or metal to brighten it up, if you wish. Drill at least one hole about 3 to 4 inches up from the base to get rid of excess water when it rains or the plants will drown. Put in some good potting soil to get things started and plant away to your heart's content. I add Shake and Feed to mine when I am planting and I get really good results.
Drill holes in the bottom for drainage maybe add a liner black bag then add dirt then flowers
I agree with Joy and Kmdreamer. Although I would put something down in the bottom to keep the soil from running out with the excess water. Screening or weed material. I would also put some small/medium stones in the bottom then the dirt. The stones allow the water to drain down into that area and the plants grow to that level but don't sit in the water and drown.