You get out of something, what you put into it! Try combining any jobs you must have to do on your days off (shopping/dry cleaning etc...) with approaching girls. Don't get caught up with coming up with the worlds best opener, just notice something about her (hairstyle, jewellery, style of clothes) and say "Hi, I just had to come up and say I love your xxxxx" and carry on a conversation.
I know some people struggle with this so rather than spend an hour online (which you don't need at the moment as you can get bogged down with too much talk of openers/techniques etc etc..), go to a coffee shop, take a seat outside (if it's too cold, take a window seat) and as every person walks by, notice something about them before they get out of sight, i.e "Hat", "rings", "bag", "necklace", "jeans", and so on...
Do this a couple of times and you'll soon find that as soon as you see someone, something they're wearing, will stand out.
This is an indirect way of opening, and is helpful as if the girl replies with a "Thanks", you can ask where she got the item/style and take the conversation from there, but remember DON'T get stuck on that topic for more than a minute, as you wan't to move things on ASAP, so just change the topic:...Wow, I did'nt know xxxx sold that sort of stuff!?, so what are your plans, this weekend? I'm off to meet some friends blah blah blah..."
Also before you start, try asking girls in shops/stores for advice on buying something, as this gets you into a chatty, social, fun state. You can joke/flirt whatever and it does'nt matter about the outcome as she's there as a kind of an unknowing "practice partner" for you.
Once you get used to talking to girls without feeling nervous, try a MORE direct opener like "Hey, I saw you from the other side of the street/shop/station & I knew I'd kick myself if I didn't come and say Hi; I'm XXXX!"
Put in the practice and your results will improve!