Gave birth to twins, they're already 2 months old. Now we're not planning…

Gave birth to twins, they're already 2 months old. Now we're not planning any more children for about 3-4 years. I don't know how to prevent pregnancy better now. I don't want to take risks because no method besides the pill gives a 100% guarantee. They say it's not advisable to use the pill after the first pregnancy. Will it be difficult or impossible to get pregnant again? What would you recommend now, as you had it? (monthly cycles haven't started since giving birth)