Short answer, sometimes. If you only intend to upload them online, then you generally don't need written permission (some websites do require it though) however it can be a good idea to get permission anyway especially if you are photographing people you don't know well or at all. This will protect you should someone change their mind about having his/her photo posted online and sue you for supposedly not obtaining their consent. Extreme scenario, but it can happen and it's better safe than sorry.

If you are going to be making any money at all, then yes definitely. Though some jurisdictions may not require it (depending on the circumstances), it is a good idea nonetheless as without it, a model can change his/her mind and take you to court over it. You have no proof he/she ever consented in the first place. Again, it's an extreme scenario but it can happen.

You need to make sure the consent form contains all relevant information regarding the shoot including whether the model is being paid, whether the model will receive a portion of the sales profits and how much, etc. If the model is a minor (where I live, under 18 is considered minor, however this varies by jurisdictions) then the parent must sign the form not the model him/herself. Make sure you adhere strictly to the consent form because if you do something outside of what the consent form allows, no matter how seemingly small and insignificant, you are in breach and the model has the right to take legal action against you

Remember, laws vary by jurisdiction, so my advice is to do some research including studying sample model release forms which can be found online easily

If you need any further clarification, just say so