It's tricky to answer your questions because it can vary from horse to horse. There's no right or wrong way to go about starting dressage.
The most important 'basic step' you'll need to kick start your dressage career is to have you horse moving softly off your leg. So you want to be able to put your leg on and get a reaction every time. Same with the rein, you want to touch your reins and have her come back to you. You need to learn to use your back to influence her also.
Once she will go and come back to rein and leg softly, you need to have her travelling off the forehand and ideally, swinging over her back. To achieve this, TRANSITIONS TRANSITIONS TRANSITIONS!! Between paces and within paces. Get walk-trot-canter-trot-walk all perfect and on your aids. Then start working on asking her to shorten her steps in trot a little by slowing your seat. Once she's shortened her steps for a few strides, ask her to lengthen them a little again by putting your leg on and holding your back.
Lots of changes of rein, serpentines and figure of 8's are both great excercises! Get her lovely and supple and listenign to your aids.
Once she's nice and responsive and soft, you can start some lateral work, but don't expect it to happen overnight, It'll take you a while to establish basic softness and response to the aids. The first lateral movement you should try is leg yield. The horse steps forward and sideways at the same time, with the legs crossing over each other. I'm not going to go into detail here about how to get leg yield as it will take far to long in this thread.
Eventually you'll be able to move onto more advanced lateral work such as shoulder in, travers etc. But it all takes time. Dressage is certainly not something that you can just one day decide to do and you'll be out there in a month pulling off all the advanced movements! Hence why alot of peopel get bored with it, the initial dressage work IS alot of circles and 'boring' area work but this is how you develop the skills to start the more advanced movements.
As for how many times a week you should school your horse, well, it's up to you. Idealling you'd ride maybe 3 or 4 days in a row, keeping the sessions fairly short and just making it very basic and positive to slowly build up your horses strength.
If you are showjumping, she will probably have quite well muscled hindquarters, but hill work is fantastic, I swear by it for conditioning toplien and building up strength in the hindquarters. Cantering up is useless, trotting is the most effective.
Best of luck and keep us up to date, more than happy to help :)