what type of tires (cheap enough for a 18 years olds paycheck) do you recommend for 15 inch rims?

And where do you buy hubcentric spacers?

And who can put them on so the ar hub centric?

ebay for the rings. under 10 bucks
and you can put them on.
They just fit over the middle of the hub on and your rim slides over them.

If they are stock rims you don't need the spacers though..
only with aftermarket rims.

and just go to a tire place and tell them how much you want to spend.
any tire is fine for you...heck walmart continental tires are decent for like 35-40 bucks a tire.

Kuhmo is good (others will disagree). try them yourself though.

those are good too.