Yes, you can. English has no rule against starting a sentence with a preposition, so your wording stays grammatical when the phrase sets the scene.
After the meeting, we'll decide. Use this placement when you want the time or place in front of the action, so your reader has the setting first.
When your opening phrase sets time or place
If the next action has a deadline or a time attached, put that phrase first. Before sunrise, the hikers reached the summit. Your reader gets the timing, then the action, and the comma marks where the opening phrase ends.
When you're setting a scene, lead with the location. Under the table, the cat slept soundly. The spot lands before the subject, so the picture is in place before anything happens.
Contrast that with fronting for emphasis, where the phrase runs straight into the verb. On the shelf sat a dusty vase needs no comma. With a longer phrase such as On the top shelf, a dusty vase sat untouched, writers commonly add one. Either way, pick an approach and keep it consistent.
Don't stack these openings. Three sentences in a row that begin with In, After or Before can feel mechanical, so save this placement for moments where the time or place genuinely matters. How often you use it is a personal style call.
One case to avoid: fronting a prepositional phrase that carries the heart of the sentence. She works in Boston works best as is, because the phrase tells us the key fact, not just the scene. Fronting shines when it's setting the stage: In Boston, she works two jobs to pay rent puts the place first as background, and the main point arrives after the comma. If the location is the point itself, keep it in its normal spot.
Whether the comma is required
For a short opening phrase, writers commonly put the comma in or leave it out, and both versions are widely accepted. After lunch we left and After lunch, we left both read as fine, so pick one approach and stay consistent through your document.
Read the sentence aloud when you're unsure. If you naturally pause after the opening phrase, the comma helps: Without much warning, the meeting ended. If the phrase runs straight into the verb, leaving it out sounds just as natural: In the shower he does his best thinking.
Longer openings usually take the comma. Because of the delays caused by the storm, our flight never left. Here the comma marks where the setup stops and the sentence proper begins, so most writers keep it.
One exception worth knowing: when a fronted phrase is inverted for emphasis rather than scene-setting, it typically takes no comma. Down the hill rolled the pram runs straight into the verb, so adding a comma would just interrupt it.
When in doubt, match the punctuation your document already uses. Consistency matters more to readers than either single choice.