Here is some general advice on how to create a character for a larp. Most of these could be adapted for other forms of fiction. They're meant for the situations when player is creating his or her own character - the most dominant form in Croatia (as opposed to premade characters).
Be aware that this guide goes into quite a detail - not all of it is necessary, and most of it is spice and fluff. As a matter of fact, it's often undesirable to go to great detail on all accounts - it can be very demanding and leave little room for interpretation. Enrichen your character to the point YOU feel comfortable with.
First of all, think of what sort of character would you want to play. What kind of interactions do you prefer with other players? Is there anything in the rules which you'd certainly like to try and do? What kind of personality will you have? Answers to those would give you your basic character. Once you've done this (and got yourselves some gear) you could be ready to go, but please consider doing more of what is mentioned in this article (or other things we might have failed to mention).
Some advice:It's probably not the best idea to play a lone wolf character. They might seem easier, or self-sufficient (which is often viewed positively), but they make it harder for you to integrate within the social environment, they limit your interactions, your experience, and the experience of others, and it's likely that you won't have such a good time. Play a socially involved character, with strengths and flaws. Get yourself involved - create content on the larp. If you're still playing a lone wolf (perhaps you desire the experience), don't complain that you're bored.
The backstory is commonly mistaken as something your character is about. Instead, it is just a minor piece of the puzzle that happens to be about your character's past. You can probably have something short and simple (like a sentence or two), or something elaborate and long. However, don't go writing an epic novel about your character - you're not expected to define and finish everything in your backstory. A long backstory can be awesome, or it can be a crutch. Leave plenty of loose ends - they provide you with the interesting thing to role-play with other players, or for GMs to insert plot hooks. Act on your backstory, but don't shove it down the throats of other players.
Different larps have different gear requirements and expectations. Clothing is probably the most important thing of all - you can get a good kit on the cheap, or invest a lot of money in it. Footwear is often neglected (though, for fantasy larps being barefoot is also "period wear", and it's free). Weapon and armor (where expected) are possibly the most expensive kit pieces - or they could become one. Dress in style that's appropriate to the larp and your character. Also, be advised that going above the minimum gear requirements is usually appreciated. Every piece of gear you get for yourself will improve the immersion of you and of other players - we're all props in each other's game.
Some advice:A larp can have an "official minimum" and "socially acceptable minimum" which is often higher. Lower gear standards are usually tolerated for new players - however, there have been some new players on larps before who had gone above and beyond of what is expected for a new or even an experienced player. If in doubt, get a better costume.
Characters with a goal to be more realistic can answer any questions about their character. There's a nice, comprehensive list for SCA personas which han help you flesh out all the details for your character (it has a medieval focus, if that is what you seek - but you can adapt it to most situations and genres).
Larp is a social activity - see the "lone wolf" stuff near the beginning of the post. Some larps will put you in a group immediately. Some groups are quite actively recruiting - a member of some group might have gotten you on a larp in the first place. Your friends might be a part of some group. Or you might want to recruit some of your buddies to group with you. If some of you are having your first experience with larps, this might help get rid of some anxiety - and provide additional social and plot elements. You might want to use some other methods (such as Ball of Yarn) to create group dynamics - indeed, a character can be largely characterized using nothing but group relationships.
The golden rule:
I'll finish this article with a golden rule for any character creation: if in doubt, consult your GM. Your GMs may know what's needed on larp - to give you more meaningful interactions, and to help ensure larp goes more smoothly. GM advice might range from certain mechanics suggestions (certain skills, classes etc.) to some personality suggestions which might create a better story. Of course, there are larps where GMs do all the characters, but even where they don't they're usually willing to help you out.
Create the character (or a few) and have some fun on a larp of your choosing! Or start/run/write a new one :)