SO LATELY BEEN WONDERING= So I’ve been wondering lately.

TO LIGHT THE SHADOWS ON YOUR FACE= To make you happy, to remove the sad expression on your face.

IF A GREAT WAVE SHALL FALL= The modal verb SHALL used to be the future form for I and WE, but not much anymore:
- We shall go to London = We will go to London.
In modern English we use SHALL for promises ("I shall be there for you"), for offerings ("shall I open it for you?"), suggestions ("shall we go?") and as an emphatic form of WILL, which is the case here:
- I will go with you= I just decided it
- I shall go with you= I promise. I’ll go with you no matter what. I’ll go with you even if I have to break the city walls to do it. (this example may be considered a promise or an emphatic future).
But here, we see SHALL used in an if-clause (a conditional clause beginning with IF). In an if-clause we use present to express future:
- if you come tomorrow, I’ll see you. (future: if you come tomorrow)
We use WILL in an if-clause meaning "want", not future:
- if you will come tomorrow, I’ll see you = if you want to come tomorrow, I’ll see you (in this example the first WILL expresses volition and the second expresses future).
So if we’re just talking about future we would say:
- if a great wave falls...
But he adds SHALL in this conditional sentence to make it sound old-fashioned, and so, more poetical: if a great wave shall fall and fall upon us...
In old fashioned English they very often use SHALL in the if-clause instead of the present to make it sound more emphatic:
- If you shall ask anything in my name, I will do it. John 16:23 (modern English: if you ask anything...)
English speakers are very familiar with old-fashioned forms because they find them for instance when they read the Bible or Shakespeare, so it is not strange to find this old-fashioned construction in a modern romantic American song (although he wouldn’t use it in normal conversation). It just sounds stronger and more beautiful (because it’s old-fashioned).

UPON= Old-fashioned form of the preposition ON.

COULD YOU MAKE IT?= To make it is to succeed. In this context it means "to survive", to carry on living.

ON YOUR OWN= Alone, without company or help.

IF I COULD THEN I WOULD= The missing parts of this sentence are in the next line: If I could go wherever you will go, then I would go wherever you will go.
Notice that he pronounces the F in "if" as a V (if I could /ɪvaɪ kʊd/). This is not a common thing, but singers often make "funny" pronunciations to make things sound "more interesting" or more romantic or whatever. Exactly the same thing happens in this song: Rule the World (pay attention to the line: "if angels cry", that sounds like "evangels")

WHEREVER= Anywhere, everywhere, it doesn’t matter where.

I'LL GO WHEREVER YOU WILL GO= The first WILL is the future auxiliary and the second WILL is the modal that expresses willingness, so this means: I will go anywhere you want to go. You can also say "I'll go wherever you go", without the idea of willingness.

WAY UP HIGH= Way is used in colloquial English to emphasize the comparative and adverbials of place:
- He’s way stronger than you = He’s much much stronger than you.
- The boat sailed way into the sea = it sailed far away into the sea
So the phrase "way up high" means "very very high".

DOWN LOW= This is an emphatic form of "down". It means "really down", "all the way down". It is the exact opposite of "up high".

I’LL FIND OUT A WAY TO... = I’ll discover the manner to...

MAKE IT BACK= If you make it back, you succeed in coming back. To "make it" means "to manage to do it", "to succeed in doing it" and it often refers to movement:
- Can you make it on Monday? = Can you come here on Monday? / Can you go there...?
- No, I can’t make it = No, I can’t come to your house / I can't go to the party, etc.  (it depends on the context)

RUN AWAY WITH MY HEART= This sentence (and the following ones) looks like an imperative, but it is possibly a present perfect (I think it makes more sense in this context): (YOU HAVE) RUN AWAY WITH MY HEART. So the form RUN could be the past participle (run-ran-run).

I KNOW NOW... = The complete sentence is: "I know now just quite, how my life and love might still go on". This sentence means "now I know more or less (just quite) the way to continue with my life".

MIGHT= The modal verb MAY, or its past form MIGHT, are used to express probability:
- Tomorrow it may rain = Perhaps it will rain tomorrow (maybe).
- I might see her at the party, but I don’t know = Maybe I’ll see her.
Not so long ago, MAY was used to express that something was probable and MIGHT to express that something was not very probable than with MAY, but today most people use both indistinctly (no difference).

GO ON= Continue.

TURN BACK TIME= Time always moves forward, if you turn back time, you go back to the past.

IF I COULD TURN BACK TIME, I’LL GO WHEREVER YOU WILL GO= In this song the conditional sentences are often used quite freely. Grammar says that the correct tense combination here should be:
- If I could turn back time, I would go... (2nd type conditional: if+past, would)
But in real life people often use all sorts of combinations in the conditional sentences. Here, he uses the past in the if-clause "if I could turn back time" because that is really impossible, but then uses future WILL in the main clause because he is now ready to follow her, not just as a hypothesis, but as a present reality. It may also express volition (= I want to go, I’m ready to go, I’m willing to go).