I don't know where you are based but I would agree that April is early enough to start planting - I usually by small plants in seed trays from a nursery and don't plant until very late April -early may when I am sure the frost is past. From experience i get the best crops from green beans, onions, leeks ( which are great as they can stay in the ground all winter and you can just dig as needed) and sweet corn although you need a good summer for this to ripen. Brassicas eg. broccoli, cabbage, kale are good until the caterpillars kill them off and I have yet to find a way to stop them.