Good for you, Laurie!

Study after study shows that the best long-term weight loss and maintenance success comes from diet +
exercise.
Yes, the scale may go up when you start exercising (though it didn't for me) and no, it probably wouldn't be muscle. It's a common misconception that we can somehow pack on several pounds of muscle the first few weeks that we're exercising. Nope, isn't gonna happen. Maybe - if we're lucky - we could add ten pounds of muscle in the first YEAR of exercise (and less after that). But none of us can add three or five pounds of muscle in a few weeks.
But don't panic!

What's happening when the scale goes up after starting an exercise prgran is
water retention. Sore muscles retain water - it's as simple as that. Eventually your body adjusts and you should see nice drops on the scale.
All this assumes that you don't start eating more because you're exercising.
BTW, you want to give your muscles a day off between weight workouts (cardio every day is fine) so if you go to the Y on M-T-W, you might want to alternate muscle groups, like doing upper body one day and lower body the next.