This simple song will help you remember a very important rule that will help you read English:

When two vowels go walking the first one does the talking

And to be more precise you must remember that:

when two vowels walk
The first one says its name!

So when you find two vowels together, the pronunciation will be the same as the name of the first vowel as you find it in the alphabet, and the second vowel... well, you can forget about it.

A /eɪ/  E /i:/  I /aɪ/  O /əʊ/  U /j u:/ 

Obviuosly this rule doesn't apply 100% of the time, in fact it's only true in about 50% of the words, but it is still good for many words. Examples:

A  (ai, ay)
day, main, stay, again, spray
E  (ee, ea, ei, ie, eo)
meet, meat, feel, peace, team, receive, niece, tied, believe, piece, phoetus
O  (oa, ou, ow, oe)
boat, road, toad, soap, loud, mown, bounce, toe, goes
U pronounced /u:/  (ue, ui)
blue, true, juice, fruit, suit

EXCEPTIONS: niece, house, mouth, eight, receive, boy, coin, moon, boot, earth, author, friend, does, liquid, build, etc, etc, etc

And one last thing to remember: if the two vowels are followed by an R, then everything changes. For example compare:

MEET /mi:t/  vs  BEER    /bɪə*/ 

PAIL /peɪl/   vs  PAIR   /peə*/

When two vowels go walking
The first one does the talking
In BOAT you hear the O and not the A

In MEAT you hear the E
The A sits quietly
The second vowel you see but you don't say

But...
Shh!

Just the two of us together
In TRAIN and PAIL and RAIN
The A speaks up; the I does not
But...
Shh! Let me explain

When two vowels go walking
The first one does the talking
In BRAIN you hear the A but not the I

In SOAP the O is clear
The A you never hear
In SAY you say the A but not the Y

But...
Shh! Isn't it neat?
But...
Shh! It can't be beat!
B-but...
Shh! It's such a dream!

Don't mean to boast
But here's a toast
We're quite a team!

When two vowels go walking
The first one does the talking
I'm sorry, number two, it's such a shame

Although it gives you pain
The rule is very plain
When two vowels walk
The first one says its name!

But...
Shh!

Yes, when two vowels walk
The first one says its name!

DOES THE TALKING= If you do the talking, you are the one who talks.

SPEAK UP= Speaks loudly.

NEAT= (esp. BrE) Wonderful, cool.

IT CAN’T BE BEAT= You can’t do it better than this.
BEAT= Defeated.

BOAST= To say good things about you with excessive pride, like feeling superior or something.

TOAST= A toast is when you raise your glass of wine (etc) in public and make a wish or a praise.

WE’RE QUITE A TEAM= We work very well together.

PLAIN= Simple, easy to understand, clear.