Umm. While I know that's fairly technical, remember that you don't have to understand all details of all financial calculations. But you could start with a rule of thumb that the lower the rate the better. If you're comparing products, just make sure that you're comparing the same thing - either get the "nominal rate" for all of them, or better, get the "APR" (Annual percentage rate).
Unless you have a bad credit history, or are looking for a very long fixed rate, 7% is very high at the moment. You should be able to get a tracker rate of a small fraction under 5% - and that will make a massive difference to your monthly payments.
Shop around, and particularly if you're not good on the details and calculations, consider going via a good broker. Have a look at the Financial Best Buys forum, where there's a list of brokers here:
I used REA years ago, and they were very good - very kind to panicking first time buyers! L D Ferguson is a regular Askaboutmoney poster, and comes across as very knowledgable and helpful. But everyone on the list has been recommended in glowing terms by various askaboutmoney posters over the years. Really could be worth it...