Learning to design isn't something that happens quick and some people just can't design, period. It isn't something that happens overnight or in a few weeks. It may be worth taking some design classes and getting into some design forums to get feedback etc, on what you have done, if you have an interest at getting better.
2. Analyse sites you like and measure the proportions, relative sizes and shapes and font sizes etc.
3. Create your site. Ask for feedback from designers.
4. Once you've gone through that process, you'll start to get an idea of how time consuming UI and front end design is.
5. if it's not for you - get some one else to do it.
Once you've got that in mind I recommend using Fireworks for mockup. It has easy to use vector tools for creating layouts, trying different colors, manipulating fonts, etc.
Skipping a good UI for whatever reason (for a web-startup) is a bad idea.
Bootstrap so you can save money for good UI.
max@maxhenderson.com