はじめして seems to be answered above, I think that's correct, so let me move on next. よろしく is informal. I usually would not use this to strangers. よろしくおねがいします is not too formal to me. I would say it's very common, so I use this most often to strangers, even if they are older than me. Then what is a formal way of saying it is that it might be よろしくおねがいもうしあげます. This one is a little too formal to me. I would be confused about how to react, in a way, if a stranger says this to me on itakli because it sounds like a business setting or a formal setting. こんにちは。はじめして。Cyrisといいます。どうぞよろしくおねがいします。 This is only one example. Also you can add more personal information of you like where you live, what you do, etc. Cyrisと申します and よろしくおねがいいたします are also OK. Sounds more foral though. How other Japanese people feel would vary, so this is my personal opinion.

I will add a little bit more to Samin. Hajimeru (はじめる) is the dictionary form of "to start". Hajimemasu (はじめます) is "I am starting". Referring to those words, when you say "Hajimemashite" (はじめまして) that means "Nice to meet you.". You basically say this only the first time meeting/introducing with someone. Yoroshiku onegaishimasu is a nuance word; meaning that it has no SPECIFIC meaning but several explanations. The word 'Onegai' (おねがい) means "a/to request". Onegaishimasu (おねがいします) is "I am requesting". Thus, Yoroshiku Onegaishimasu means "Please take care of me from now on" OR "I am requesting your kindness/care". Usually it is used after you introduce yourself, just like in the example above in the answer given by Samin. Also, it can be used after you request someone else to do something for you; they will be very happy to hear if you use it at that time! Do some research on Google for more example words to be more familiar with it. Since this is a nuance word I hope you could get the basic jist of what it means. My advice=> if possible, try watching as many Japanese movies and dramas as possible with English subtitles. This will really be a massive help. がんばってね！＾＿＾

hi for starting greeting in formal or informal way you can use 始めまして、(hajimemashite) = nice to meet you よろしくお願いします。is formal and よろしく is informal. you can use these phrases to show your kindness and ask others to be friend or kind with you or help you. after saying "hajimemashite" and introducing yourself you can use them. for example はじめまして Cyrisです。よろしくおねがいします。 so if you do not know your language partner i suggest you using formal japanese. at the end of the letters or e-mai you can use "yoroshiku (onegaishimasu) as "regards" to show your politeness. がんばってください

I am from china Ihelp friend thanks