海報...

I would suggest you to use Adobe InDesign for the poster's layout, but if you don't have it installed, I think word would work ok as well if you are just making simple ones. I would not suggest you to use paint since the fonts are sometimes pixilated.
If you wish to include images that's not done by you, make sure they're either clip arts or they're not protected under copy rights. There are quite a few clip arts available in microsoft's software and official sites. You may type clip arts and search online for useful ones too.
here're some of the sites you might find useful for images

http://office.microsoft.com/en-us/clipart/default.aspxme
http://www.clipart-graphics.net/
http://www.freeanimatedclipart.com/

(一)
你要整一張what類的海報?
(二)
你用word做海報較好

簡單的可以用photoimpact.....MS Word.... MS Publisher
pro小小...可以用photoshop, illustrator or indesign....