Money put into an ISA earns tax free interest from the day it is opened. So there is no particularly advantagous point in the year to put the money in that will lead to an increased interest within that year. Nonnie is right, the sooner the money is invested the sooner it earns interest, but that applies to any savings, not just ISAs
The ISA year runs, give or take a few days, from April to March.
In the year that runs from April 2015 to March 2016 you can invest up to £15240 in an ISA. You can invest as little or as much as you like, up to that limit, you can pay into it in one lump sum, several lump sums or on a regular monthly basis. The choice is yours.
The money can be invested as cash, or in individual shares or through investment funds, where the fund's managers draw a wide range of investments into one fund with the intention of reducing risks and having a more resilient fund should the fanacial situation get rocky.
Each ISA lasts one year. You cannot add funds to it after the April year end. You have to open a new ISA with a new allowance for the new financial year. However you can move the money in an ISA from one product to another. For example if you saved into a cash ISA with the Nationwide last year and discovered this year that the Halifax paid more interest you could transfer the money in the ISA from the Nationwide to the Halifax.
If you are a non tax payer there is no advantage in saving money in an ISA.
While I agree with Jinglebellfrocks that the best ISA is an investment ISA. You shouldn't invest in the stockmarket in any form unless you have a good sum of money saved in cash form in a bank, building society or similar body and I wouldn't buy investment based products if my savings were less than £20,000 and even then, unless you are very knowledgable I would seek professional advice from a reputable firm of investment advisors, like Hargreaves Lansdowne before buying investing.
Finally never ever buy any financial product, within an ISA or not, that you do not fully understand. It is better to appear stupid and to tell the advisor you do not understand it and will not buy it what you do not understand than pretend to understand and then lose all your money.
All the above is, of course, my personal opinion.