You can't as the cheapest membership is €10 so you'll need to topup by at least another €5 to buy membership.
If you have a current membership and have used up your calls allowances then buying a new membershio won't help as the new membership will not start until the current membership has eventually expired.
In that case you need to buy an add-on (e.g. mobile or landline mins - depending on what sort of calls you need to make) to tide you over until the current membership expires.
Hope this helps.