Who needs a visa? How to get it? Embassies & Consulates, of the country abroad and in the country.
If you are a E.U or USA citizen a visa is not required. Check the Israeli embassy in your country prior to travel.
If your passport has stamps from other Middle East countries be prepared for a long wait and questioning.
Remember why you are there and remain calm and relaxed in response to their questioning. If you have any plans regarding visit to Palestine (excluding the Betlehem trip), it is best not to share this information with the Israeli authorities. If you write about such trip in this site, it is best to hide your full name/phone number. Israeli authorities are notrious about banning/Denaying entry of anybody who is into Palestinian friends/World peace/Human rights/Studying arabic/See the other side of the occupation. If you bring with you a laptop, do not keep any palestian contacts on your hard disk - instead mail it to your email account. Try not to bring any other item which will make you suspicous of ever talking to a Palestinian or having the slightest wish of meeting one in the future. and keep calm!