There is no easy answer, Teetime. This thread shows some of the issues. We have to wait, first of all, to see which candidates are standing in our constituency. You would never know it from the media, but we can only vote for our own local representative, not Mrs May or Mr Corbyn. We don't have a president. Voting Green may not be an a option for you.
More waiting — until we can see the manifestos. Remember, these are only lists of wishes and dreams. No Party can guarantee to do what it says. And when you look at the two or three main policies, decide based on those, because those are all that may possibly be carried out. Don't reject any party on the content of page 50! You will never agree with everything any Party says.
Then there is the question of sleeping well after the election. If you want to change the system, you may have to watch someone you didn't support go to Westminster, but be sure that the numbers of votes cast for each Party will be given lots of publicity. Compare the number of votes cast for each party with the number of MPs elected under their banner. It is clearly unfair. We need to change the system, but in a carefully considered way, not to AV which we wisely rejected. In a marginal seat, put you convictions on one side, and vote for the best candidate who has a chance of stopping what you DON'T want.
We only have one vote. We have to use it as best we may and hope that someone in the madhouse of Wesrminster may notice that (sadly) most of us vote 'not-***' -- fill in the relevant blank for your area — not positively for anything. No wonder things are in a mess!
Personally I would vote for anyone who didn't keep churning out the nursery rhyme chants they seem to think pass for policy. I have stopped listening to 'strong-and-stable' and 'the-many-not-the-few'. Just like 'take-back-control'!