First you have to check which providers are available in your postal area. Do a search in Google (vergelijk=compare) [google.com] and fill in your postal code. Most sites will give you a list + references of available providers.
Tiscali has a very nice offer in the Netherlands right now. You can pay 18 months ahead. For example you pay €360 for a 4mbit connection for 18 months. That is €20 per month which is dirt cheap. You will have to pay the full amount of €50 for six months after, because the contract is 2 years. Please keep that in mind. The Tiscali ADSL connections are pretty ok, but their helpdesk is horrible.
XS4ALL has very good uptime and srvice (the always win the "consumentenbond" competetions), but they are quite expensive. Demon sits in between. They are more expensive than Tiscali and cheaper than XS4ALL, but their service and uptime rocks.
Dont's go with SpeedsXS or new providers in general. Usually the providers that have been active for 5 or more years are a lot more reliable, although they are a bit more expensive.
Do a Google search for "ADSL vergelijken" or something. You will find plenty of sites that offer ADSL comparisons.