There are two ways to grow onions, from seeds or from sets. I have grown them both ways and in my opinion you get bigger onions from sets, however you get a bigger choice of variety from seed.
Growing onions from seed means starting them off in a warm environment either an unheated greenhouse or a warm window sill. Then in spring once the seedlings are roughly as thick as a pencil they can be planted outside in rows about four inches apart in well draining soil and water them in dry spells.
Onion sets are easier, an onion set is basically a small onion that has been grown commercially and then you just plant them outside in rows about four inches apart in well draining soil, just leave the tops of the onions above the soil and water in dry spells. Onions take about six months to grow. There are some varieties that can be planted out from September and over wintered these are ready to harvest in May. Other varieties are planted out in spring and are harvested around about September/ October. Most over wintering varieties cannot be stored for too long although check the packets as new varieties are being developed all the time, this year I am trying a variety called Shakespeare which they claim store well. I am also growing red onions for using in May. I plant my onions and garlic in beds alongside each other and in between I have planted some winter density lettuce.