1:48a

How do You know me?: This is a real question. Nathanael asked Jesus how he knew what sort of person he was. It is implied that he had never met Jesus before.

Here are other ways to translate this question:

How can you know me? (Phillips’ New Testament in Modern English)
-or-
How do you know about me? (New Living Translation (2004))

1:48b

Before Philip called you, I saw you under the fig tree: Jesus was pointing out that he knew something that a person did not know without help from God. This was a sign (evidence) that he was from God. He knew what Nathanael had been doing before Philip had invited him to go and meet Jesus. John does not say what Nathanael was doing under the fig tree, but in some languages you may need to include some information. For example:

I saw you sitting/resting under the fig tree.

Here is another way to translate this sentence that may be more natural in some languages:

I saw you in the shade of the fig tree.

the fig tree: This phrase refers to the specific fig tree that Nathanael had been under. The fig tree was a common tree in Israel. It normally produced small fruit called “figs” that they could eat. If speakers of your language are not familiar with the fig tree, here are some other ways to translate this phrase:

the tree with a fruit called “fig”
-or-
the fruit tree