At this point you should be reasonably stuck into your summer fitness sessions. Before you get entrenched though, we should probably address your attire. That old tee you wear to the gym might feel comfortable and help you feel at home in unfamiliar surroundings, or perhaps it says 'WHAT THE BEACHES WANT', or some other terribly humorous (read 'douchy') comment that you think ups your ante in the social hierachy of gym politics. It doesn't. What it does do is retain water i.e. your sweat, which weighs you down and constricts movement. Especially in the case of any cardio you're doing to slim down and tone up, this will have a negative effect on your performance.
You need to invest in performance apparel, specifically designed for this purpose. Your tee should be scientifically designed to fit and move with the contours of your body. The best performance tees will feature ventilation and/or increased breathability, fabrics that wick moisture away from your body to the surface of the garment, with fast drying properties.
This Performance Run Tee from Craft is a good example, and is available at a great price point.