If you’re 18 do can your parents still claim you on their taxes? And if so how does that effect my tax return

It depends.  Your parents can claim you as a dependent as a "qualifying child" if you:

1. are under age 19 at the end of the tax year (24 if a full-time student, or any age if disabled):
2. lived with your parents for more than half the year;
3. did not provide more than half your support; and,
4. did not file a joint tax return (except to get your withholding back). 

If your parents can't claim you as a qualifying child because you are "too old", they can still claim you as a dependent as a qualifying relative if:

1. your gross income for the year is less than $4,200; and,
2. they provided more than half your support for the year.

If your parents claim you as a dependent, you must file a tax return if you are single and any of the following apply:

1. Your unearned income (e.g, interest & investment income) was more than $1,100.
2. Your earned income (from your job) was more than $12,200.
3. Your gross income was more than the larger of—

• $1,100, or
• Your earned income (up to $11,850) plus $350.

If you are married, or 65 or older, or blind, the filing requirements are slightly different.  This Table 2 from IRS Publication 501 gives all those details.

Even if you aren't required to file a tax return, you may want to do so to get a refund of any taxes you had withheld.

Table 5 from IRS Publication 501 is a good overview of what it takes to claim a dependent.