There's a great free keyword tool on Google Adwords that is really helpful when trying to pick the best keywords for your campaign.
Yes, this is a great tool. But Adwords beginners should be very careful with it.
One of the most common mistake I have to fix when managing a new Adwords account is way too many semi relevant keywords thrown into random ad groups.
(The other is lack of negative keywords... which can easily eat up half your daily budget on non relevant clicks)
The best place for beginners to start is to choose keywords that are "phrases that your best customer would type in if he was actively searching for what you sell" and grow out from there.