Zaineb
I recommend you take a look at the IIBA's BABOK (v1.7 is avaialble from their website) and specifically at the chapter called Varification (5.11.) It's only a couple of pages long and will answer several of your questions.
As for your specific questions above; you should aim to get signoff as each document is completed. Each document in a formal project environement is treated as a specific artefact that needs to be managed, and approvals are one of the management activities.
However, different organsiations have different cultures around who signs what. In some organsiations the busines stakeholders sign everything, and in others only the sponsor and pm do. In some organsaitons the approvers are limited to IT once you get into solution specific documents and in others everyone is watching everything throughout the process.
You should speak with your Project Manager and line manager about your approval plan for each document, especially on your first run through the process.
In fact you could even pre-define who will be on what document and incoporate it into a requirements communication plan (see the BABOK for Requirements Planning ch 3, and Requirements Communication ch 6.)