If it is to post in the forum, you don't have to resize it.
If you need to resize it, you can use many programs - or download irfanview from tucows, or from a google search. It can crop, resize - and do all the basic things very quickly and easily.
If you want to post the picture. You can:
click on your gallery, all users have one. It is the camera icon beside your name.
upload the picture onto the server. When done correctly, it will show you the little picture.
left click to make it bigger
right click, properties
copy the location (highlight and ctrl c)
paste the tag url inbetween the - *[img]* tag and the *[/img]* and put it in your post where you want the picture. don't include the *