I think the only way to get less nervous lectoring is to 1) pray to God for him to help you and 2) lector a lot.
If possible to do a dry run at home the day before your lectoring day where you just stand up and read the appropriate reading out loud, that could help you get familiar with it in advance so you won’t stumble over words when you do it at church.
Regarding appropriate outfits, I’m assuming from your profile name and past posts that you’re a girl.
First, find out if your church has any kind of dress code for lectors. If there’s a dress code, then follow whatever it tells you to wear.
Second, make sure that your clothes aren’t too tight or too revealing. Any top you wear should have a modest neckline, a back, and sleeves (short sleeves okay), and shouldn’t cling to your bust. You can also wear a blazer or a cardigan over a blouse or dress. Any skirt you wear should reach your knees or below. If your church allows female lectors to wear trousers, then wear a nice pair that aren’t too tight. Wear nice shoes with a low or medium heel that you can walk in easily.
DON’T wear any of the following: a camisole or spaghetti strap top, a sleeveless or backless top or dress (unless you wear a blazer or sweater over it), any top that shows your midriff, anything low-cut, anything clingy or tight, yoga pants, athletic wear, athletic shoes, shorts, any skirt that hits above your knees, sandals (unless they’re the dressy kind).