It's a big subject - we can probably give better advice if we know:
- what decade of life you're in
- what types of assets you have, such as house, 401(k), collectibles, straight investment accounts, CDs, and so on, since they have to be handled differently
- what type of life insurance, if any, you have
- the relationship between yourself and those to whom you wish to leave assets: spouse, children, siblings, friends?
- the relationship between yourself and those to whom you don't wish to leave assets
- your marital status
- your (very approximate) net worth
- if you felt you could share it, your state (assuming you live in the US)
For example, in Oregon, even if you have a will and a living trust, if your liquid assets (like a checking account) are more than (something like) $70,000, the will goes through probate, which is a hassle. It will save your heirs a lot of trouble if you re-title the checking account with the cash to be in the name of the trust, rather than yourself personally.
It can get very tricky and there are a lot of "gotchas". So unless you have a very simple financial situation, the best investment is often a lawyer specializing in wills/trusts/POAs. A good lawyer will think of all kinds of scenarios and account for them. For example, if you think that a child might contest a provision of the trust, he/she can put in a clause that anyone contesting receives nothing. You should be able to get this done for less than $1k. Our situation is complex, and we got a trust set up and wills and POAs and reviewed healthcare POAs for ~$1200. Well worth it.