Birthday Present Help.

smartphone :P

Probably a dildo

Are we mind readers ? how we suppose to know personal tastes ...

check what she likes and think about it, wtf dude

Get rid of something that begins with "v"

No, just joking.

very easy puzzle.. Solved.

This

Giftcards or just a straight card with money. 18 y/o love money.

Well girls like clothes and jeweleries so i would suggest you look towards those.
Obviously I dont mean underwear or weird jewls

Every girl loves these... really, they do

If you really have no idea, go for an accessory that almost every girl wears. A silver bracelett. They are usually quite cheap and I haven't seen a girl to mind receiving one as a gift. Moreover, you could spend a bit more and buy one that stands out, considering it's a gift for your niece.
Cheers, mate !

Never met anyone, male or female, who said no to a box of their favorite candies.

get her a gift card. if she has a smart device, then get her a gift card for it, to put apps on it. otherwise pick a popular store in your area that most people like to shop at. if your in the states, id suggest a Macys gift card. this might not be something that is super personal, but if you have no clue, its better to get something like this cause then she wont be disappointed if you pick something else that she isnt interested in. if she is super eclectic about what she wears & is anti name brand clothing & likes thrift shopping more, then id suggest an Amazon gift card. cause you cant go wrong with it. Amazon tends to have everything.

some clothes or a gift card

As a girl myself, I'd say you can't go wrong with money.

It's hard to advise you with such little information. Maybe you should club together with someone to buy her something "bigger".

You could always try this

Take her out on a vacation to a nice place she always wanted to visit if the money is not a problem.

A car

the humble weekly. good for all ages and genders. I even played them before and would apriciate.

a nice watch and some cash

As a 24yo woman, I can tell you that all I've wanted for my birthdays/namedays/christmases for the past ~10yrs is cash. Or what's more practical, money straight to my account. (edit: oops, I see you're not a fan of the cash gift. Sorry, scratch that then.)

I guess gift cards work too, but they're kind of limited. My top places to get gift cards to would be steam, my favourite online clothes retailer, or a good makeup retailer with lots of options (like Sephora if you have it).

If you want something more personalized, my best advice is to just ask her. Presents don't need to be surprises, everyone's happier if you get her something she genuinely wants. If you do want a surprise, uuh.. idk man, tell us more about her? I see you mentioned she's girly/feminine, so I think you can't go wrong with something makeup related. If a gift card's not personal enough, either get her a single big makeup set, or a subscription to a monthly beauty box. I also always appreciate a new bottle of my favourite scents, but for that you'd need to know what she likes because you can really mess up with a perfume that doesn't suit her. One my of three favourites is the classic Lolita Lempicka (Le Premiere), I think a 18yo feminine girl would probably like that, if you can't find out what her personal pick would be.