(This should be a comment, but it's too long.)
For a very brief overview:
- The lowercase forms developed out of Mediaeval handwriting, becoming fixed in their modern shapes with the invention of the printing press.
- The uppercase forms go back to Roman inscriptions.
- The Romans borrowed the alphabet from the Etruscans, who borrowed it from Greek colonists, with various minor changes along the way (such as Δ tilting sideways to become D).
- The Greeks borrowed it from the Phoenicians, adding a few letters of their own invention, and repurposing some extra consonants as vowels (which weren't indicated in Phoenician).
- The Phoenician alphabet evolved from the Canaanite alphabet, which evolved from the Proto-Sinaitic script.
- At this point the details get less clear. The Proto-Sinaitic script was probably derived from some form of Egyptian writing, though it's hard to say anything for certain about it; not very much writing in this script survives, and the actual meaning of the writing we have is still debated.
- If it does come from Egyptian, Egyptian hieroglyphs are derived from earlier pictograms being extended to write arbitrary words. For example, the word for "mouth" was something like raꜣ, so a picture of a mouth was used for the "R" sound in words having nothing to do with mouths (such as the preposition meaning "to", which is harder to draw).
And there's a lot more detail in each of these phases that I'm glossing over. If you can narrow down your question a bit and explain exactly what part you're interested in, you'll get better answers.
