Learn English – reported speech and usage of “will” or “would” — “I told him that I will (would?) be there at three o’clock on Friday.”

The word you're looking for is shall; but you've got it backwards. Shall is the strong (more definite--this isn't about tense) form of should. So, it should be more like this:

"I will go to Europe." — He said he would go to Europe.

"I can play the guitar." — He said he could play the guitar.

"I **shall** go to college." — He said he [should] go to college.

Here's another way to phrase that:

I *think I should* go to college.

I'm not sure whether you're asking about American or British English, but in American English, shall is very uncommon. It's much more frequently used in the UK.

Americans would be more likely to use should for both forms; but we sometimes use ought to, must, or have got to instead.

If the situation reported is still valid at the time of reporting, backshifting is optional. See Swan (2005) Practical English Usage, pages 275-6, for several examples.

John said that he would give me a pen. Correct in all situations.

John said that he will give me a pen. Possible if John has not yet given the pen to the speaker.

S2: I wanted to let you know that he had sent you an email. Correct in all situations.

S1: I wanted to let you know that he has sent you an email. Possible if the sending was so recent that 'you' may not yet be aware of this.