I went to thailand when I was 21 on my own. it went like this: "Oh, I have a layover in bangkok on my way home from OZ...I'll extend that to 4 weeks, and I'll travel ALL over! from bangkok down to the islands, two weeks of island hopping, and then slowly make my way back through the country! it's gonna be EPIC!"
reality: I went from bangkok down the islands, first stop ko panghan, and I never left that place until I had to go back home. it was EPIC nonetheless. My reasoning was that in ko panghan, I'd found paradise, and anywhere I'd go, I'd only wish for it to be like ko panghan. so no use in even trying when I was so goddamn happy in my hammock down there.
anyway. bangkok was really, really scary, but it won't be as scary for you because you're a) shorter (I think) and b) not as white as me. it took me a week to figure out why the hell everyone was staring at me like I was an alien, and it was because I looked like a paper-white giant to them. which is their beauty ideal. I didn't really do much in bangkok other than check out their dirty, scary markets, which were great fun, and buying a thousand bootleg cd's and conterfeit nike shoes.
ko panghan though, man, that place is awesome. I went down there by bus, which I wouldn't recommend doing. took forever and the aircon left me with a cold. it's one hellish trip. down there, I stayed at a hotel in the northwest of the island, away from the only "big city" (forgot the name) where everything was happening, and it was serene and beautiful. the cost of living was the same as the cost of cigarettes in germany, so I lived like a fucking king. We could even add our weed to the hotel bill. Oh man. the three famous big parties, half moon, full moon, and black moon, are all in ko panghan, and tourists and thais from all over go there. all three were fucking fantastic. full moon is on the beach, and all the bars and restaurants participate and there's about 2000 people partying. one of the others is in the jungle, and the other one on a deserted beach up north. it's unbelieveable, really. so all I did was go to these parties, then take a week to recover, and then start again. then eat green curries, and fruit salad, and drink that sticky sweet coffee. back to bangkok, I took and overnight train, which was much better because they just left the doors open instead of turning on the aircon, and I had valium. and they gave me food. and I met a hot canadian. so take a train, by all means.
useful tips: always cover your shoulders, take off your shoes whenever you enter any building, including restaurants. carry tissues with you at all times, they don't have toilet paper anywhere but in the really touristy areas. make friends with a protective white male to feel secure - mine was an irishman named carl. do not smoke weed anywhere at any time, although I did all the time and nothing bad happened and it was the best weed ever, but carl was arrested for drug possesion and that was no fun at all. all the nice places are crowded with tourists, but there's tourists and there's backpackers - go where the backpackers are. eat, eat a lot. I went days with only eating pineapples. because man, I'll never forget how they tasted there. it's another world. try and find a family-run hotel - they'll just include you in the family. and stare at all the trees in the jungle for days on end, you'll never see anything like it anywhere else. and don't drink the red bull - I did, but I was in an experimenting-with-drugs-phase, and that stuff got me high like nobody's business. I once fell off the back of a scooter because I'd had one red bull.
most of all, everybody tries to tell you how insanely dangerous it is to travel there by yourself. I was on my way home from a year of travelling when I went, and I could not give a shit about danger anymore if I tried. I think that's what made it so much fun. do not be afraid. it's not more dangerous than any other place in the world, it's just different, and that's where the fun lies.
also, since you're doing europe, too, I really hope you're gonna go east, too. go to russia.
I've always known I'd be a bank robber. So judge all you want, ladies and gentlemen. Because you never did become an astronaut.