One thing I'd say is drink water - and lots of it. First time I went to Marmaris, it was my first holiday abroad and without my parents. Thought I could drink beer all day and night long with just a few bottles of water. Ended up coming home a week early after spending about 5 days in bed dehydrated.
You're supposed to drink 2 litres of water a day here in the UK, so make sure you're drinking at least that in Marmaris. What we tend to do is get one of the big 5 litre bottles of water and keep it in the fridge in the room, going through it between two of us every two days. Then when we're out and about during the day and night, we order a smaller bottle (half a litre? The coke bottle size ones) with every drink we get. A bottle of beer and a bottle of water.
Don't mean for this to put you off, Matty, I just want you to have a great time!
Other thing is don't worry too much if you think you're upsetting someone who's trying to get you into their shop or restaurant. I know this has been clamped down on, but it still happens. If someone is pestering you to go in, just say 'No thank you' and walk on. Simple.
There's so much to tell you, but last thing I'd say is leave a tip and you'll get great service - and I mean GREAT service. We tend to stay by one restaurant during the day, using their sun beds by the sea, ordering food and drinks from them, etc and tip as we go along. By the second or third day, they know we'll provide decent tips and you'll soon find yourself having beds reserved for you and drinks brought to you within a few minutes of ordering them
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Ha, I don't know what I'm more excited for - my holiday or you going for the first time!