That's absolutely normal. The advice given in books ranging from Dr. Benson, author of "The Relaxation Response," to the Dalai Lama is to not fight those wandering thoughts. The more you fight them, the further away you get from a still mind. Accept them and let them pass. If they're strong, think about them for a minute, then promise yourself to spend time with those thoughts later.
That'll always happen, but in time they'll come up less often won't bother you much.
I don't know what method you're using or how long you've been practicing, but, in the beginning, you might try having or focusing your attention on, say, soothing music, a candle flame, a scent, or a religious item that's meaningful to you (a small statue, mandala, a cross, Star of David, the OM symbol, etc.).
However, whether religious or secular, the purpose of meditation is to withdraw from all external sensory input (sound, sight, etc.). So the next step is to count breaths: On each exhale, count 1, then, 2, and so on, up to 10 (I like only going up to 4 -- whatever works!)
Or you could try a "mantra." There are prescribed mantras in Sanskrit (Hinduism) and Pali (Buddhism). You could choose a verse from a sacred text or a short prayer. You could even use a word or very short phrase of your own choosing: "calm," "still," "ocean," "love," etc. It's best to *think* the word as you exhale, but in some practices, the mantra is repeated aloud. Saying it aloud -- softly -- helps focus your mind.
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