Foil balloons need to be filled with helium or air. They fill easily through a small self sealing valve and, if helium is used to inflate them, they float for days, as they hold the helium in much longer than ordinary latex balloons. If you fill them with air, you will need a straw or something similar to push through the self sealing valve in order to blow them up, withdrawing the straw when they are full.
There's a great tip for you when decorating with foil balloons. Make sure that you tie the end of the balloon below the valve, as if they start to go down before you want them to (and they should last for days) then you can always top them up again with helium or air because the valve will continue to work effectively in most cases many times. I've known people who've used the same foil balloon for three different parties. I have to say they don't look at their best after several uses, but they are really easy to top up.
If you aren't able to get helium or you don't want the fuss of using it, then there are simple things you can do with foil balloons filled with air. They are very easy to stick to the walls so that they look as though they are floating, because unlike latex, they are resiliant, and very unlikely to burst, so the pull of a bit of blue tack when sticking them to a wall or removing them later is not likely to damage them.
If you are going to make table displays with them, we recommend that you only use two balloons. We recommend odd numbers with latex as a result of their size and the way that they sit with each other, but foils behave differently and they are also bigger, so three foil balloons look to heavy above a table. They are equally effective as the centrepiece of an otherwise latex balloon display, and this way you can get in a contrast in colours if you wish.