Buy a sewing pattern. It will take you step by step through the process. That's how I learned.
There are 'easy to sew' patterns that don't use a yolk and usually don't have cuffs. Think of summer beach shirts. A traditional dress shirt takes a little longer and will fit better. If you are a complete beginner an 'easy to sew' pattern wouldn't be that difficult.
Unless you know the order to assemble the pieces you are going to have a hard time. There are cuts that need to be stabilized with stay stitching while you are working and some that need to be eased. It really isn't difficult. Just takes a little time the first time through.
Once you know the process and the pieces then you can start improvising.
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If you can already use a sewing machine with skill, simply unpick an existing shirt to use the pieces as a pattern.
Cut out your fabric, and sew back up the same way you unpicked it.
If that is beyond your skill, and instructable probably won't help you, and you need to take some sewing / dressmaking classes.