Employment contracts form the contract between you and your employer with a view to outlining expectations and processes for resolving differences. For example, your employment contract will set the expectation that in return for your salary, your employer expects you to assist with securing any IP you generate for the business' benefit.
You should be given time to consult before signing an employment contract, typically at least a few days. Seeing a lawyer is a good idea. If you don’t, at a minimum, show it to your parents or guardian for a view on whether the contract is reasonable (or not).
All contracts are negotiable. You must have a contract which means both parties must ultimately agree, however this does not mean you have to accept the contract as is.
If you choose to change an offered contract, you are making a counter-offer. At this point you no longer have a job offer, the offer is now with the employer whom does not have to accept. If the employer counter-offers again, you have a job offer again. If they choose to not respond. you have neither a job nor a job offer to consider.
To change a contract, rule a line through each line of text to be deleted (the underlying text should still be readable) and initial the change adding any extra text or notes needed to clarify your intent. Show (or discuss) your changes with an employment lawyer. Employment and contract law is complex with its own lexicon of terms, legal precedence and common understandings. Be aware that without specialist advice, it is unlikely your changes will affect the legal interpretation you intend.
Some elements are fundamental to employment contracts (in legislation) and very important to the employing business. For example, it is unlikely that employers will agree to you retaining the IP to products they wish to commercialize. Demanding that you do, is likely to result in the business choosing to employ someone else. When presenting a counter-offer, do so in person where you can 'feel' how receptive your employer is to the change and can take immediate feedback which may mitigate your concerns or see them addressed in other ways.