First of all, don't worry - reading texts which were written four hundred years ago was never going to be easy! Even the most acclaimed scholars had that moment where they first opened their copy of The Merchant of Venice and thought, huh?! So to begin with, don't be too hard on yourself. It takes time to get your head around language that you're not used to, and that's ok!
Let's have a look at The Merchant of Venice. Don't worry if you don't know the story, you don't need to for this explanation. Have a look at this quote, where Bassanio is describing a painting of his love, Portia:
Bassanio:... Here in her hairs
The painter plays the spider and hath woven
A golden mesh t'entrap the hearts of men
Faster than gnats in cobwebs. But her eyes-
How could he see to do them? Having made one,
Methinks it should have the power to steal both his
And leave itself unfurnished.
(Act 3, scene 2, lines 123-129)
First of all, and this is the most important part of this whole answer, read, reread, and reread it. Yes, it's fairly time consuming, but trust me - if you keep going over a sentence that you don't understand, you're far more likely to capture a glimmer of meaning than if you read it once and write it off as being too difficult.
If you still don't understand it after reading it a few times, highlight and look up all of the words you don't know. This actually has two functions - if you go to highlight some troublesome words and find very few or even none, then you can be assured that you will be able to work out what your text means - it's written in language you understand, after all! In this case, however, there are a few words which could potentially trip you up, so let's have a look at them.
hath - this is simply another way of saying 'has'.
t'entrap - words are often put together like this or are shortened by people like Shakespeare, and it usually serves to keep the rhythm in what they're writing. Think of the apostrophe as hiding (usually) a vowel, and you should be able to work it out! So in this case, Shakespeare means 'to entrap'.
unfurnished - looking up this word gives you two results:
1. (especially of a house or flat available for rent) without furniture.
'an unfurnished apartment'
'he is unfurnished with the ideas of justice'
We're all rational people, so we can work out that the second definition is the most relevant when it comes to the quotation we're studying.
Defining all of the problematic words and terms used in a text doesn't necessarily solve its complexity straight away, however. The last stage, and the hardest one, is to think. Think about what the author, poet, or playwright is actually saying. So let's have another look at the quote, in two parts this time, and including our newly defined words.
'Here in her hairs / The painter plays the spider and has woven / A golden mesh to entrap the hearts of men / Faster than gnats in cobwebs.'
We can now clearly see that Shakespeare is using a metaphor to describe the painting's artist drawing Portia's hair, and that men become attached to and ensnared by it.
Now, the second part of the quote.
'But her eyes- / How could he see to do them? Having made one, / Methinks it should have the power to steal both his / And leave itself not supplied.'
Here we can see that, in some cases, simply changing the word to its dictionary definition doesn't work. This is where we have to really think and try and work out what he's actually trying to say.
Let's start with 'But her eyes- / How could he see to do them?'. We can change the vague word 'do' to 'paint', for it is the artist who is doing the action. So it turns into 'how could he see to paint them?', which makes things a little easier. 'Having made one, / Methinks it should have the power to steal both his' - read this out loud, try to work out what words could be missing, and reword it in a way which you understand.
'Having painted one [eye], I think it should have the power to steal both [of] his [eyes]'
Finally, 'And leave itself unfurnished'. Well, let's think about this logically. In this hypothetical situation, the artist has painted one beautiful eye, only for it to steal both of his eyes. Only one eye has been painted. The eyes are therefore unfinished. So the quote becomes:
'But her eyes- / How could he see to paint them? Having painted one eye, / Methinks it should have the power to steal both of his eyes / And leave themselves unfinished.'
It doesn't have to make complete sense, it is essentially a work of translation after all, but the point is that you can read and understand it.
So, to reiterate:
1) Read, reread, and reread the bit that you're struggling with.
2) Highlight and look up all of the words you don't know.
3) Think logically.
And remember, don't worry about it not making complete sense at this point. If you decide to analyse the quote further then it will become a lot clearer. All that matters now is that you can read and understand it.
I hope this helps!