As a new Virtual Assistant (VA) you willl often have problems with cash flow until you're up and running and making a regular income.
As a new VA though, when you take on your first few clients and until you have built up a relationship with them it might very well be worth asking for anywhere between 50 and 100% upfront for the work that you are due to undertake. While this may seem stiff it is a lifesaver for many a VA.
While not guaranteed a client who has to part with some or all of their money to get a project off the ground are likely to be more reliable to work with than one who is concerned about paying you anything upfront.
The main thing though is to trust your instincts and if something doesn't seem right about a Request for Service then before you get involved in the nitty gritty of preparing a quote why not run it past some more experienced VAs and see what they have to say, they may well have had dealings with the perspective client and be able to give you some good feedback on them.