Ummm.. Well it depends on your own skill level with sewing etc. if you’re already good at sewing or propmaking or whatever you can start with more difficult projects. I’m gonna assume you’ve got the basics down and nothing more, just to make things easier.

• Well, this is kind of obvious but don’t do anything that you know is way above your skill level. Especially for a first cosplay (I’m speaking from experience here)
• Just pick a character you like, preferably one with a simple outfit with clothes you can mostly get from stores, flea markets, your own wardrobe etc. Just something you can easily make yourself or alter from old clothes. 
• Avoid using certain types of fabrics like crushed velvet, broadcloth and cheap satin. Also, don’t use fabrics like pleather and vinyl if you have minimal experience with sewing.
• Get a wig because your own hair will most likely ruin the whole cosplay unless you know how to style it or your character has very short hair or a hairstyle that’s difficult to pull of with a wig (like a tight, high ponytail or a mohawk). You can find plenty of cheap, good quality wigs on ebay.
• Don’t start the day before the event you’re planning on wearing the costume! That will just result to desperate crying under the stairs, no sleep and a shitty costume you hate with burning passion.
• And remember that your first costume is not supposed to be your best. Cosplay is all about trial and error. You try a lot of different methods during the process of making a costume, some work and some don’t. The whole point is to learn from your mistakes and improve. Seriously, I made every possible mistake a cosplayer can make when I started and I haven’t made any of those again. Well, except for the starting late thing which I still do but yeah…
• Just… don’t take things too seriously and have fun :D

I guess that’s a few things… I’ve never been asked anything like this so I don’t really know what I’m actually supposed to say. Anyway, don’t quit just after one bad experience in cosplay because I can guarantee that this whole hobby is just one huge bad experience with the occasional good things that come with it.