Once your child can confidently say the right sound for some letters, they will begin to be able to read short words. (Children are usually taught the letters s, a, t p, i & n first as this allows them to immediately begin to read many 2/3 letter words such as 'sat' or 'in'.)
The way that we teach children to read simple words is to ask them to say each sound in the word one after the other quite quickly in order to hear what the whole word says. This quick sounding out and blurring each sound together to hear what the word says is called blending. We are literally asking children to blend the separate sounds together so that they can hear the complete word (e.g. "c-a-t...cat!"). This is why it's important for us as parents to say each letter sound correctly and in it's shortest form. What I mean by this, is that we need to say the letter sound rather than the letter names (i.e. for Aa, we say the sound "a" not the name, "ay") and we only say the sound, we do not add any additional sounds. Therefore we say "m" not "muh", "d" not "dugh" etc. This is important, because otherwise children trying to sound out and blend the word 'mum', for example, would hear "muh uh mugh" which doesn't actually sound like the word. We need to use short clear sounds instead (i.e. "m-u-m, mum").
If you would like some more information on blending and how to support your child learning to blend, please take a look at this Instagram post. It contains lots more information and you are able to leave a comment to ask me any specific questions if you'd like to.
Remember, children take different amounts of time to learn and develop. Even if your child knows all their letter sounds, they may not yet be able to blend and 'hear' the word just yet. It takes lots of practise and playing around with sounds. We cannot and should not rush them through the process. It will come!