A solo female traveller in Algeria will usually garner a lot of attention, both positive and negative. Being asked a zillion times a day whether you're married and have children and why you're travelling alone is not unusual. It's wise to have some stock answers and perhaps an imaginary husband. While this attention can be tiring, it also has some advantages, and doors will open to you that would likely never open to a solo male traveller. A foreign women travelling alone will likely find herself with constant invitations by local women to join them in whatever it is they're doing.
How you dress will have a big impact on how people perceive and approach you. Dress conservatively with long skirts and long sleeves. A head scarf isn't required although in the south it could be worth wearing as much for the sun protection as for the modesty value.
In cases where you are subjected to unwanted come-ons and suggestive lines, trying to ignore them is one possibility. If that doesn't work, it's a good idea to turn to other people (men or women) for help. They will almost certainly come down like a tonne of bricks on the perpetrator.
One of the more awkward situations a woman travelling alone might have is if she hires a guide. Most will be completely honest and trustworthy; others might have more romantic ideas. Use your own judgement when hiring a guide, even if it's only for an hour or so.