To start off, go for about half the maximum time you are allowed on them (for some its 10 minutes, others it is half an hour, they will advise you on this in the salon), about 3 times a week. After about a week you should start to see a difference, albeit subtle.
Always leave at least 24 hours between sessions and use a cream - they should sell them in the salon. There are loads of different types some have a bit of fake tan in and some don't but they all help protect your skin a bit and tan a wee bit faster. Also like with any kind of sun exposure remember to moisturise inbetween sessions to help stop your skin from drying out and prolong your tan once you have it. xx