How can I stop my mind wandering during prayer?

You can't stop it completely, but you can reduce it a lot. A wandering mind doesn't cancel your prayer, and almost everyone struggles with it. Prepare before you start, pray words you understand, keep your space plain, leave your phone in another room, and gently bring your attention back each time it drifts.

Why the mind wanders

The Prophet ﷺ explained that when the call to prayer ends, Satan returns and whispers to a person during prayer, making him remember things he had not thought of before, until he doesn't know how much he has prayed (Sahih al-Bukhari 608). So if your shopping list suddenly appears halfway through al-Fatiha, you're in good company. Allah describes successful believers as those who are humble in prayer:

قَدْ أَفْلَحَ ٱلْمُؤْمِنُونَ ٱلَّذِينَ هُمْ فِى صَلَاتِهِمْ خَـٰشِعُونَ "Successful indeed are the believers, those who humble themselves in prayer." Quran 23:1-2

That focus (khushu) is a goal you grow into, not a switch you flip. If you feel little at all, see What if I don't feel anything when I pray?

Before you pray

Handle your needs first. The Prophet ﷺ said there is no proper prayer when food is served or when you feel the call of nature (Sahih Muslim 560). So eat if you are hungry and the meal is ready, and use the toilet first, unless the prayer time is about to end.
Take a minute. Make wudu calmly and stand still for a breath before the takbir.
Silence your phone and put it out of sight, not just on silent.
Pick a plain spot. Once the Prophet ﷺ prayed in a patterned garment, looked at its marks, and asked for a plain one because it had distracted him (Sahih al-Bukhari 373). A mat with a busy pattern or a cluttered room can do the same.

During the prayer

Understand what you say. Read the translation of al-Fatiha and a few short surahs, then recall the meaning as you recite them.
Rotate your surahs. Reciting the same short surah every day makes the mind run on autopilot. A new one makes you listen to yourself.
Slow down. Pause after each verse and in each position. A rushed prayer is a wandering prayer.
Look at one spot. Keep your eyes on where your forehead will rest.
Return gently. When you notice you've drifted, just come back to the words. Noticing is the skill, so don't scold yourself.

What if I lose count?

If the wandering makes you unsure how many rakat you've prayed, base it on what you're sure of and finish with two prostrations. See What is sujud as-sahw?

Be patient with yourself

Don't expect to be fully present in all five prayers on day one. Pick one prayer a day to give your full attention to, perhaps Fajr or a quiet one at night, and let the habit spread. Even a prayer where your mind wandered is still valid, your effort to improve counts, and a little progress is worth more than a perfect plan. See How do I memorize short surahs for prayer?

Key points

A wandering mind doesn't cancel your prayer.
Prepare: handle your needs, silence your phone and choose a plain spot.
Understand and rotate what you recite, and slow down.
When you drift, return gently without guilt.

Tip: Before the takbir, say to yourself, "I'm about to stand before Allah." That one sentence is often enough to quiet the mind for the first rakah.