Yes, you need to pressure wash first. Usually just water and some pressure, 1500-1800 psi, is all that is needed. It all depends on the condition of the stain and the type of new stain you will be using.
You want a clean and uniform colored background for the new stain. Exterior wood stains are available in clear or translucent, semi-transparent and solid. Clear is just that and solid looks like paint. The more color the better it will be at hiding the existing background color.
If the old stain is stubborn you can use an injectable wood cleaner/brightener. These chemicals can be injected directly through the pressure washer if it is equipped with a chemical injector. Most are biodegradable and harmless to plants and do a good job at removing old wood stains.
You will need to let your house dry for 2-5 good drying days, lots of sun, depending on where you live. The more humidity the longer it will take for your house to dry out. Example; the south west with 20-30% humidity will take 2 days, down south with 70%+ humidity will take 4-5 days.
Caution; Using a pressure washer on soft wood, like cedar, is difficult for the novice. It is very easy to cause damage to the wood. Start with low pressure 1000-1200 psi at first and get used to this tool. If excessive pressure is needed use the wood stripper, this is safer.
How long will an exterior wood stain last? I hear this a lot. It depends on many things. How clean is the wood, the type and quality of the stain and the amount of color or pigments it contains.
General rules are;
1-The better the preparation the deeper the penetration of the stain into the wood. The deeper the penetration the longer it lasts.
2- Oil based stains penetrate deeper into wood but can oxidize faster than many acrylics. Acrylic stains are more UV resistant but they can peel if there is excessive water in the wood.
3- The more color or pigments the longer it will last.
4- Cheap stain burns off faster. Quality costs much more than $80 per 5. A true super quality stain like Sikkens Cetol Log and Siding is as much as $500 per 5 retail. This product does last, you get what you pay for.
If you go cheap you will be restaining in 1-2 years. Mid-grade quality, like an emulsified oil, 3-5 years. And going with a very good products from Sikkens or Messmer's you won't need to restain for 7-8 years.