Bajo is an adjective that means short or a preposition meaning under, so you need a noun after, like El perro está bajo la mesa (the dog is under the table).
De is a preposition meaning origin or possession. Yo vengo de España ( I come from Spain), el abrigo de Laura (Laura's coat).
Del is the combination of the preposition de + the article el, only for masculine nouns. El color del arcoiris (the colour of the rainbow).
Hope it helps :)