You can use Microsoft Word or PowerPoint to create your poster. These are not graphical layout applications, but they are adequate in most cases.
In PowerPoint, create your poster as a single slide. You can set the page size when you start using File > Page Setup, so if you want an A1 poster (594mm × 840mm), you can specify this before you start (there may not be an A1 option, but you can enter the dimensions manually).
PowerPoint also allows you to add guidelines to help you line up the poster elements. Use View > Grid and Guides... and tick the Display grid on screen box.
In Word, create your poster as a single side of A4. You can always scale it up when you come to print it.
Word does not have guidelines as such, but you can get a grid by showing the Drawing toolbar (View > Toolbars > Drawing) which will probably appear at the bottom of the screen, then from that toolbar choose Draw > Grid... and tick the Display gridlines on screen box.
In both applications, use the Drawing toolbar to add text boxes to the screen. This allows you to control the way the text is positioned on the page.
When the poster is designed, you should convert it to PDF for printing, using PDF Creator or Adobe Acrobat. The conversion process can be problematic: edges of words and images may be cut off near to the margins, images may appear degraded or misshapen, poster elements may have shifted and become overlapping. However, by ironing out these problems at the conversion stage, you avoid nasty surprises later when you come to print it out. When the PDF looks good, you can be pretty confident that the printed version will also be OK.
Test your poster early, and regularly, by converting to PDF and printing to A4, to make sure you're not storing up layout problems that will be difficult to correct later on.