What Do I need to Build a good portfolio to send to agencies and apply for Jobs?

you need the basics, you need to show how you look like and it's as simple as that, 1 or 2 shots of your face, 1 or 2 shots of you standing straight in lingerie or swim wear to show How your body looks like not laying on bed or a couch, when it comes to photoshop or what is known as 'air brushing' it should be very subtle and minimal and that's where the ability of a good re-toucher comes in (fixing little imperfections without replacing the entire skin texture or over the top obvious re shaping, if a potential client sees a photo of you and it's heavily airbrushed the first thing they will think is 'she has a very bad skin' or there are other problems.
In my professional opinion I believe a Model should show how versatile she/he can be and that comes after you have obtained the basics. modelling is a big and over saturated market and you need to stand out from the crowd and you'd want clients as they flick through ports for them to stop at your port and want to see more and get their attention and think 'I like that'. Quite often I look at photos of one model then I think 'no she can't give us the look we need' then I look at more photos and change my mind and think 'ok no she can she has great facial expression'. 'the more looks you can do the wider range of clients you can attract cater for'. Also the more work you do the more experienced you become and your portfolio will reflect that, 'a portfolio is a forever ongoing project'. more experience and better portfolio means so many things; you know how to pose and show your best and you'll give the photographer more usable photos ratio in a shorter time span with less effort, and most likely you're not a nightmare to work with as a few clients or photographers have already worked with you, eventually you'll use one or 2 photos only from your best shoots rather than 8 or 10 photos from a single shoot.

Modelling isn't about looking sexy or pretty, sometimes it is and sometimes it's about looking exactly the opposite, it's trully a skill and very demandng unlike people's general perception.

Eeveryone in the industry has an opinion and will lecture you on what you can do and can't do, Don't be put off by what a single person says, I know a model who is 5'2ft and she turns in a few grand's a month and worked with top photographers in the UK and abroad, how does she do it! she learned where's her market is and she knows how to 'market' herself and she 'Networks' with the right people and most importantly she is very 'determent' and she looks after her health and Body as it's her tools. I come across so many models and the truth is even with an over saturated market with models very few of them have what it takes and it has nothing to do with their looks, it's their attitude towards modelling and where their poses comes from, Equally there's nothing guarnteed like in every other career, it's a personal choice to try or not but all what I would say is if you decided to get into modelling you need to put 100% in and do your home work.