I've just had a good question from a first time voter via email, so I'm sharing the response here:
Firstly, thanks for emailing me and giving me the chance to respond. I'll give you two answers - the issue based one and the technical one.
If you vote for something you don't believe in, or don't much believe in, you are probably wasting your vote. If you want society to change and new ideas to develop, red or blue are the two parties that have dominated politics in this country for a century. On the issues, if you feel that the Greens represent you, even if a Green MP doesn't win in your constituency, every Green vote will give weight to the actions of Caroline Lucas and any other Green MPs that get elected to Parliament. If they represent the views of 1 million voters, even with just one or two seats, you are being represented.
The second answer is technical. In Liverpool Wavertree, you are living in one of the perceived safest seats for Labour. There is no chance of a Conservative (or even Liberal Democrat) winning this seat. Labour will probably be disappointed if they poll less than 60% of the vote here. I'd recommend reading my blogpost on the forecasts for Liverpool constituencies written in March http://www.petercranie.blogspot.co.uk/2015/03/electoral-calculus-election-forecast.html
Last week, LBC conducted a poll in Liverpool, with 38% of people saying they would vote Labour, the Greens in 2nd place with 9% but with over 30% of voters undecided. In last year's council elections we were the second biggest party in terms of vote share. Increasingly Liverpool is likely to be a Labour v Green contest, even under our outdated First Past the Post System. So you can vote however you want here, with no danger of the Conservatives winning.
A final point. Caroline Lucas was our only MP in the last Parliament. She won her seat with 31% of the vote in 2010. In the 2005 election, we had won 22% and in 2001 just 9%. If those people in Brighton in 2001 and 2005 had decided to vote tactically for the least worst option in those years, we would never have had the political credibility to win the seat in 2010. So how you vote in this election doesn't just matter for the result now, but also for the future.