Hi, sheep! We get quite a few budget travelers coming to this forum without checking that we are really a wilderness trekking site. I'm so glad you checked! Some things we do have in common, and most of us have traveled at one time or another.
What you really want is a budget travel site like Lonely Planet's Thorn Tree (although it's not as good as it used to be). There are other sites with great information, mostly related to the various popular travel guides (Let's Go, Rough Guide, Rick Steves). While I very much disliked Rick Steves' guidebooks, his suggestions about packing light are excellent. In fact they agree with the lightweight backpacking philosophy espoused here. Most of going lightweight is not about buying expensive gear but about taking less "stuff."
You'll find that thanks to modern quick-drying fabrics, you really don't need an enormous backpack for traveling. Traveling light is a lot more fun and means you usually don't have to check your bag, thus saving the extra fees, waiting at baggage claim and possible pilfering. It also avoids some of the horrible things baggage handlers do to checked baggage! I looked up this excellent article for you:
Pack Smart and Travel Light.
There are articles on backpack selection on
the home page of this site, left hand column which should help you. Just remember that a travel backpack, unlike the lightweight packs many of us use while hiking trails, must be stout enough to stand up to those baggage handlers (I've seen them throw bags to the pavement, missing the people and cart that was supposed to catch them).
The most important aspects of any backpack are fit, fit and fit. The pack must fit your body, it must fit what you carry inside it, and it must be comfortable for you to carry with the load you put in it. You'll find fitting suggestions in those home page articles I mentioned.
I personally prefer a convertible travel pack for what you're doing. I have one with internal stays and a hip belt which, when on the back, carries like a regular internal frame backpack but whose belt and shoulder straps can be zipped out of sight behind a fabric panel. This not only protects the straps from getting caught on baggage machinery, but also allows me to walk into a hotel carrying a normal-looking suitcase should I decide I want a break from the hostel routine (or find, as frequently happened to me, that all the hostels are full and a hotel is the only choice).
As we often say here, Your Mileage May Vary.
Best wishes for a great trip!