To change your picture find a nice picture you like on the internet. Then click on your account, click change avatar, touch the pencil, the it will say browse, Then you will browse your pictures for the one you want, then click on the picture and it should change it, If not it may be too big, too small to upload then you may need to find adifferent one.

When you make a skin. Try using a variety of colors. Some that most people don't commonly use. But since your new to skinning try to practice with different techniques. Maybe adding lines or "strands to the hair"

As for originality, try to make skins that are completely original. Here the flower crown is good, but has been done a lot. So to make better skins try something no one has done before! For example, If you like biscotti try making a funny skin made of biscotti.

As for the clothing and shoes, try to use some shading. Shading will add dimension to your skin and make it look better. Also, try not to use a hair shading technique that you would commonly see.

Like what was done here. Since your new it would be a good idea to develop your own techniques. Over time you will become better at this and your skins will start to look better. Soon you'll have your own skinning technique. Another good idea would be to not use clothing you commonly see in a minecraft skin. That will make your skins look better too. Take some examples from real life to help you.

Secondly,( you did not do this is just a warning)
try not to recolor skins from other site and call them yours you could get banned or lose your posting privileages. Its better to make your skin from scratch than to take someone elses hard work.

Thridly, Don't get discouraged If your not popular at first don't quit. I've seen good skinners quit because they weren't recognized. The thing with that is that they needed more practice or had another reason. Remember not every skin you make has to be posted.

A great way to become more recognized is to get active. Comment on others works. Not only skins. Things like blogs, projects, and resource packs.

Also, going to the chat will also allow you too make friends, but don't advertise on their or you'll be moved away from chat. and also try to encourage others and be nice.

and last but not least, use a good skining system. the better the skining system the better quality your skins will come out. I use McSkin3d. you can download it here. :
http://www.planetminecraft.com/mod/mcskin3d/

If you have any more questions feel free to ask me :3