l really wanna climb Rainier, but its so damn expensive with a guided group.If l take mountaineering course, could l safely ascend the easiest route?

Yes u can climb Mt. Rainier if u have the required skills & apply for a permit to climb. Generally speaking the Disappointment Cleaver route is the easiest & most popular. If ur not going to get a guide than u must go with experienced climbers as part of ur rope team. Get involved in a climbing club & network. Or just pay for safety & go with a guide.

You should absolutely enroll in a mountaineering course & take the safety issues very seriously . Climbing Rainier is dangerous & u should assess the risk involved. When climbing at high elevation, every task, even taking a step, becomes more difficult & requires more energy. Give yourself time to train.

l have summited Mt. Rainier but it took me 2 tries. The first try l made the right decision to turn back down due to inclimate weather & mostly for the consideration of a rope partner who had altitude sickness. You have to be OK with not making it to the top if the weather is not right. It can be a wonderful experience if u approach it the right way.

Read this book by Mike Gauthier, it is a must:
http://amazon.com/s?ie=UTF8&search-type=ss&index=books&field-author=Mike%20Gauthier&page=1

Yes, u can climb Rainier on ur own,. but it is not really any cheaper in the long run.

As stated by the other responder, take a course. Courses r offered by a few clubs that will be less expensive than guided climbs/seminars:

http://mountaineers.org/
http://boealps.org/
http://wacweb.org/default.view
http://bushwhackerclimbingclub.org/

These courses start at $400. This does not include any of the equipment that will be needed to climb. Cost of gear is probably around $200 if u buy used & rent what u can not buy.

The downside to these classes is that they almost all start in February/March & take about 3 months to complete. l believe the Bushwhackers start later in the year & take less time.

On the other hand, a guided climb of Rainier will cost around $800 with RMI, including everything. This year, the park service has allowed more guding companies on the mountain, so there might be a less expesnive alternative.

It is alright by me. But, sorry. l have a really pressing need to rearrange my Q-Tip collection that day and can not accompany you.