lol you will get sucked in.
Whats the details of that tamron lens, looks like a decent bit of kit?
Yep aperture is the size of the hole the light goes in. If your taking photos which require the background to stay in focus you will, as a rule of thumb, use a higher f number. However the larger the aperture the more likely you are to get lens flare (which you can see in the photo of my mate).
This is my favourite lens without going into the L series canon professional lenses http://www.amazon.co.uk/Canon-EF-Zoom-lens-3-5-5-6/dp/B00006I53S/ref=sr_1_1?s=electronics&ie=UTF8&qid=1430164282&sr=1-1&keywords=canon+28-135mm+is+usm+lens I pretty much dumped all my other lenses for this one and my 50mm F1.8
The 18-55mm lens you are using is a good all rounder and has image stabilisation which is a must on a camera with a cropped, slow sensor. It will be suitable for most photos. If you are wanting to go down the route of getting shallow depth of field then you will need to always use your 18-55mm lens @18mm as this is where the aperture is the largest. It will be hard to achieve though.
Keep an eye on gumtree for lenses as bargains do come up from time to time. The 28-135mm IS lens is a great lens and you can pick them up at a decent price. The 24-105L lens is much more expensive and there isn't that much difference between them, especially when being used on a cropped sensor (aps-c cmos)