sigs

I wanted to know about softwares which can tell me what keywords should i use for my upcoming new site so that they are ranked higher in a search engine. my old site is getting good amount of traffic though i didnt do such search for keywords but that was my first site and didnt know much about keywords and stuff.

peeling

Wordtracker - Keyword services for professional search engine optimization

No other keyword analysis software comes even close to wordtracker. Tells you the best keywords related to your site, best keywords that are easiest to get a ranking for and much more.

There is another, and more costly way. You sign up for a Google adwords campaign and bid for keywords ( meaning you spend money ), and find out which keywords cost the most. So now you know which are the best keywords.

Melos

I second wordtracker. Here's a huge list of free SEO tools you can try out - 156 SEO TOOLS - The Tool Place - Free SEO TOOLs - All In One Place!