I found when I was much younger it was quite easy to get involved in games like Dungeons and Dragons where your character has a background that you create and they are as customisable as you want to make them. I also found it extremely easy to get into a roleplaying stereotype.
One day I spent a lot of time going through people's text game backgrounds and histories and found the most common ones to date (remember that not a lot of people stick to a roleplay they basically set for their character.) They are:
Abandoned by Mother / Father
This classic roleplay type plays on the story that they were abandoned by a member of their family. This is not to say that only their mother / father would be the one abandoning, but they are the most common. Father had to go to war and never came back, while the mother turned to the streets to feed all the hungry mouths and just one day never returned. This makes perfect roleplay for a typical loner, do-not-trust-anyone character type. If you're okay not having a lot of friends and roleplaying the lone ranger in a text game, it's quite a good choice.
Remember that this character has abandonment issues. They feel like everyone around them in the MUD is setting them up for the ultimate let down. Your character will likely get emotional easy and often overreact to someone being gone for even the shortest period of time.
Betrayed by a Lover / Friend
This roleplay personality comes from any typical Romance novel you have read where the heroine / hero must rise above his mistrust in the opposite sex. Either they had a relationship go sour or they were dropped off in the middle of nowhere with no shoes to walk home on, or they were left to foot a bill they did not have the cash for. Either way, this type is quite an easy role to sit into roleplaying.
Your character will probably be weary of everyone. in the text game Your character tends to get emotional quickly, but anger is their tool of choice. This is a good character type to pick up if you have played a lot of JRPGs, as this tends to be common.
The Lone Ranger
This roleplay personality delves from all of the old Wild West films. This character is the bad boy in all respects, tends to scoff at the law and generally woos women and / or men quite easily! They give off this aura of naughtiness that most people want to make the attempt to 'tame' them. It's an easy character to pull off in a text game if you do not want to roleplay at someone, but you want people to come to you. This, however, comes with its' own obvious challenges.
Remember that your character often does not ask for things, he tells. Your character will roll into town on either a steed or his own worn out leather boots and pick apart the other people surrounding him in the text game with a quick wit and honest tongue. Most thieves will play this part, because they do not take any nonsense from anyone and could care less about making friends.
The Misunderstood Soft-Hearted Shmuck
This character is often played by a lot of youngger kids who come into the text game roleplaying environment a little green. They do not want to step on the toes of any individuals but they do want to try to roleplay something, even if it means they tend to annoy everyone else in the text game more than anything. This character tends to be overly forgiving and not really one with a backbone.
In order to roleplay this character you need to really turn off your trigger switch for angry / happy. You need to generally throw yourself into everyone's conversations without needing context and have the innate ability to answer any question asked even if it is not directed at you.
The best thing to do when roleplaying in a text game is to generally avoid the stereotypes. Try to create your own roleplay and make your character as if he is a living, breathing entity – not a familiar tale borrowed from somewhere else. Perhaps do a T-chart of what his / her likes or dislikes are.
Also remember that your character should always be separate from you so that you can really have fun with events and people. Just because your favorite color is blue does not mean your character's favorite color has to be blue. Really work with things that are given to you and be sure to try and remember everything that happens!
Here are a few examples I have of how to create your character while you're playing in the text game!
Keep a journal of everything that happens. This is the easiest way to remember names while you are getting started in the roleplaying world. Did someone give you a necklace? Take note of the name.
Stick with a choice you make. I had a character that hated the color yellow because it was her sister's favorite color. She would not accept daffodils as presents and scorned it when a lover or someone attempted to give her yellow-colored clothing. They had a reason for the hatred and she stuck with it!
Have reasons for why you do things. If someone tells you to do something you feel your character would not do, do not do it – even if you're friends with said person out of character. Why would a priest be friends with a necromancer? Just because you met them once and they helped you out does not mean you'll be best buddies. You need a motivation. If someone raids your city during the day, do not go hunting with them at night! It reflects poorly on your roleplaying abilities.
Refrain from giving out of character reasoning for anything. "Please give my friend back his stuff because he went to the bathroom and left his character logged in." I'm sorry, but you need to be held accountable for your actions and anything you do inside of a roleplaying world is considered in character. Man up to your mistakes and take responsibility. Do not let it happen again.
My advice in the end is to make a character as if they are a real person. Yes, people change but it will take a drastic event to cause someone to go from truly evil to truly good. Always have a reason for why you are doing things with your character, even if you will never tell anyone this reason. Every character needs a motivation! Choose yours and stick with it!