welcome to the sight
on i tight budget maybe a FOWLER tank is in order this way you can get away with just a cheap light to view your tank.
Need: stock or other cheap light fixture, heater and thermometer, HOB or other device with filter and carbon for mechanical/chemical filtration, Live rock/sand for biological filtration, source of salt water and test kits, and fish food.
fish depends on whats available but a deginner with a 15 the cheapest thing to do would be to add 3 small damsels - after the cycle of course.
normally i'm not a big fan of damsels because they will get big, mean, and ugly if you keep them alive for a very long time, but in a beginners set-up on a limited budget they are not a bad choice - colorful, hardy, will be very active and entertaining to watch, and of course cheap. buying three will keep them from fighting all the time, and have enough rock to have hiding spots.
a little beter, but more expensive option, would be to buy two small tank raised clowns - they are equaly hardy and entertaining but in general not as mean as the damsels. you can also add inverts like snails, crabs, shrimp, and echinoderms.
then if in the future you want to upgrade your lights for corals you may want to return the damsels to the store. but this is just one option of millions - after setting up your tank and getting it cycling with live rock - take the time to research what YOU like and make your decisions then. spending a little more money in the beginning on GOOD choices saves a lot of money in the long run!