Если у меня будет время, я смогу позвонить тебе позже.

Если means “if” and introduces a hypothetical or uncertain condition. You use если when you’re not sure the condition will happen (e.g. “if I have time”).
Когда means “when” and implies the speaker expects the event to happen for sure (“when I have time”). Switching to когда would sound like you’re certain you’ll find the time.

Будет is the future tense of “to be.” Here you’re talking about a possible future moment (“if at that future moment I have time”).
If you said Если у меня есть время, you’d be talking about conditions in the present or in general (e.g. “if I have time now/in general”).

In Russian, possession is expressed with у + [person in genitive] + есть (or its future/past forms) plus the thing in nominative.
So у меня (lit. “by me,” genitive of “я”) + будет + время (nominative) = “I will have time.”

Мочь is imperfective (“to be able”), and its future is periphrastic я буду мочь.
Смочь is the perfective counterpart (“to manage to”), and its future simple form is смогу.
In this sentence we want one specific future ability (“I will be able to (once) call”), so we use the perfective смогу.

After a modal verb like смочь, Russian takes an infinitive.
We choose the perfective infinitive позвонить because it expresses a single completed action (“to make a call” once). The imperfective звонить would emphasize the process or repeated calls.

The verb звонить (to call) takes the person being called in the dative case: звонить (кому?).
So тебе answers “to whom?” — “to you.”

Yes. Russian allows both orders. Putting the main clause first doesn’t change the meaning: Я смогу позвонить тебе позже, если у меня будет время.

Позже means “later” (at a later time relative to now).
Потом means “then” or “afterwards” and is a bit more vague about exactly when.
In many contexts you can swap them, but позже is more precise about “a later point.”