1-get paid online (software and settings)
Include a line on payment terms in your invoice. If working by transference in Europe, furnish BIC and IBAN. The BIC is the SWIFT that American agencies sometimes ask for.
Note that cheques are still a preferred form of payment in the US.
2-work safety ( how am i sure i will get paid after my translation is received)
As far as ProZ.com is concerned, check the outsourcer's rating on the Blue Board, but do not limit yourself to that. You can also find out from other payment practices list.
3-Waiting times for payments
May vary, but may depend on the outsourcer. As Tayfun points out, there is an EU directive on payment at 30 days, but national implementation of this still varies.
3-Affordable legal help for translators
Here on ProZ.com, this is a common question on the language forums (local level), since this will vary depending on where you are. For example, some of our Spanish translators are also lawyers and can give initial advice for their jurisdictions.