Welcome to TP Gina
Generally speaking, a slightly longer lens is preferred for portraits. On a crop camera, which I'm guessing you have, that's usually between about 50mm and 100mm. This gives a little bit more distance between you and your subject, which is less intrusive and the perspective looks more natural.
Ideally, you would combine this with a low f/number, like f/1.8, which gives shallow depth of field, throwing the back ground out of focus. You have to be very careful with focus though.
Sigma does an 18-50mm f/2.8-4.5 with OS (optical stabilization), or an 18-50mm f/2.8 which is rightly popular and is quite a good length for portraits and will at least give you a go at shallow depth of field at f/2.8. However, assuming you've got the kit lens, particularly the Canon IS version which is very good, I'm not sure that either of these lenses will take you a big jump forward. Most people I think would say use you longer zoom at 70mm, and the lowest f/number, for portraits, and see how you get on. Take it from there. Maybe a 17-85mm f/4-5.6 IS would suit you; nice lens with very handy range, better made than the kit and has IS. There are a couple in the For Sale forum for about £220 I think.
Just throwing a few ideas out. TBH I think you might try to explore a bit more with the lenses your've got, find their limitations and see more clearly what it is you want to do. There is tons of choice out there, several different solutions for most things, and it's very easy to make a costly mistake.