When raising seed fund by angels how do evaluate the cap of the comapany

This is a very tough questions. The short answer is "it depends", on a great many things. Also, it is always negotiable, and I try not to offer a number first. However, in most situations this is not possible.

There are some benchmarks. For example, there are valuation services that can provide some guidance, basically using market comparables. There is enough data on deal flow to make this work, in my view. Consider stage of development, industry sector, and quality of deal. It costs something, but is often needed for 409A purposes as well.

One other approach I find useful is the so-called venture capital method. You can find this in textbooks, one I use (I teach at GGU) is Valuation by S. Titman. In simple terms you project a future enterprise value, using the standard financial projection framework, discount that back to the present at the (very high) VC discount rates, and use that as the pre-money value.

I hope this helps, it is an important and difficult subject.

Let's address each question individually:
1) Don't be too surprised that no one wants to be first - they are worried that no one else will join in and theirs will be the only money in. You can resolve this by agreeing to keep the first money in escrow until a minimum amount is raised. Also, in order to get "someone in first" you'll want to focus on the individuals or groups who know your industry / business best and get them to be your 'champions'. Once you have a reputable angel or group in, getting others to follow is easier.
2) Since you are the "seller", you really can't expect the buyer to make the first offer on valuation. As hard as it is for an entrepreneur to do, you'll have to come to grips with setting a valuation. At your stage there isn't a good formula for determining value so you need to find out the terms on which other companies in your area have raised seed financing - this becomes your guide. In general I can tell you that you can expect to give up between 20% and 40% ownership for the seed round and that the ultimate valuation will be determined through negotiations. Where you end up on that range, will depend on a number of factors including, size of market, competition, your past experience & depth of understanding of your customers and the market that you're addressing, your ability to sell your vision and more. Sometimes early stage companies use a convertible debenture for the seed round and offer a specified discount on the next financing as the conversion pricing. This can be a good instrument if you and the investor can't agree on a valuation but it also has potential dangers so get good legal advice before you sign a term sheet.

Good Luck!