"What are the foundational truths of Jesus we must believe to realize our salvation?"

I'm going to shut up and let the Scriptures speak for themselves. Maybe I/we should do this more often.

The apostle Paul tells us the truths he first accepted about Jesus, and that we must accept in order to realize the salvation Jesus has already bought for us. He reminds a group of believers what Jesus did for them (and us, and the world) that is at the heart of their faith in Him. 1 Corinthians 15:1–4 (words in italics are mine),

Now I am making known to you, brethren, the evangel (good news) which I bring to you, which also you accepted, in which also you stand, ² through which also you are saved, if you are retaining what I said in bringing the evangel to you, outside and except you believe feignedly (not truly; fake belief). ³ For I give over to you among the first what also I accepted, that Christ died for our sins according to the scriptures, ⁴ and that He was entombed, and that He has been roused the third day according to the scriptures

—Concordant Literal New Testament

It's perfectly fine (and recommended) to reread these powerful words—maybe several times—to really sink them into your heart.

1. Christ died
2. Christ died for our sins according to the scriptures
3. He was entombed
4. He has been roused the third day according to the scriptures

Believe in Him.

Sincerely,
Wes

P.S. If you'd like to read a much more in depth treatment of thē good news where I don't shut up, read The good news of Jesus and how it benefits you