To develop vocabulary
1) Keep your sentences simple and short. Repeat the target word in lots of different environments and settings. A child with difficulties learning vocabulary will sometimes need to hear a word 20 times before they truly understand it.
2) Give additional information to reinforce understanding of vocabulary. Sound and meaning clues are often the best.
For example: Target word apple.
Meaning clues: You eat an apple, the apple is green, the apple is round, the apple is yummy
Sound clues: Apple has two claps (syllables), apple starts with 'a'
3) Incorporate play when learning vocabulary. If your child understands how an object is used he/she are more likely to use the word functionally. So if wanting to learn the word 'ball', use a ball functionally in lots of play activities and everday situations.
You can always find more tips, strategies and advice on how improve speech, say more words, use sentences etc here.