Hi I want to get cheap EU power in FTB mindcrack 1.4.7 I was thinking of a watermill powerplant but I don't know how to make it. I don't have many resources to spend so solar power is not an option. does anybody know something that is easy to build and does not require much tin/iron?
Although watermills are fairly cheap to make they provide very little power. I am fairly certain of this assumption since you are talking about a 'watermill powerplant' that your intention is to use multiple watermills to power your desires. In the case of owning multiple watermills, you probably wont want to fill them manually since that is way to time consuming so i'm guessing you go for the automatic (unmanned) method.
Unmanned: 0.010 EU/t per block of adjacent water. The maximum output of an unmanned Water Mill is 0.25EU/t (3x3x3 cube of water minus the mill in the middle, and 1 piece of cable to direct the energy = 25 water tiles).
As stated on the FTB wiki a single unmanned watermill surrounded by the maximum amount of water blocks can provide 0.25EU/t. Since the watermills have no internal power buffer it is suggested that you feed the watermills into some sort of power storage device (the cheapest one is being the Batbox).
Also stated on the wiki is the fact that the Batbox can accept a maximum input of 32EU/t. This is also the maximum amount it can output.
Short side-note: Your machines will soon start to require more than 32EU/t so even if you should decide to get a lot of watermills it wont last to long.
If you decide to go for the eco-friendly method of using only water power you need (besides a lot of space) a total of 32EU / 0.25EU = 128 water mills. Yeah that are a lot of water mills (also not so cheap anymore). Although its a very time-consuming, expensive and not the most effective method of gaining power, it is a viable one and so you could decide to choose this.
An alternative to using watermills are a few generators. Now i don't have hand-on experience with your mod-pack but a quick google search showed me that the modpack contains Steve's carts 2. This means that you could fully automate a tree-farm providing you with unlimited wood (which you can obviously use to power your generators). Although this method does require a decent amount of minerals to start with its not that bad.
Generators provide a maximum of 10EU/t so four generators are enough to power your early needs. Upgrading your tree-farm also means you can upgrade the amount of generators and so increasing your power output.
and many many more methodes. You'll be surprised how much you can learn by simply googling.
an alternative to water mills is the wind mill (4 iron and the generator)
they produce more per tick but require a free area around them for maximum efficiency, and the higher they are the more power they generate.
don't put it too high though because they might break.